Kamina: Simon look! It's a hot spring! Ohh... Ohh, a hot spring! A veritable smorgasbord of everything a man's heart truly desires and craves! A building that's sole function is to accommodate to a man's passion, fiery spirit and raging soul!
The cast finds a reason to spend an episode at a hot spring (onsen, [温泉]) — a popular type of Japanese resort/attraction similar to communal bathing. It's the Japanese equivalent of a European spa town. Naturally this provides an excusable opportunity for Fanservice if a show is so inclined, although some shows aimed at younger kids will use toweled characters. Some series that aren't geared towards younger viewers will broadcast with towels, but remove them from the DVD version as an incentive to buy the DVDs.
In milder situations, this is merely equivalent to a bath scene where the episode slows down so the characters can relax and think about the events of the day. Often a trip to an onsen will lead to characters accidentally seeing each-other in the nude. Or intentionally seeing each other in the nude. Or falling onto each other in the nude... whatever they want, really, as long as someone ends up naked.
Occasionally, there may be a little ping-pong playing afterwards, and the characters will likely wear yukatas for added fanservice.
The low-budget version of this is the sento [銭湯] episode, where the characters go to the public bath house together. Usually, it's because there's a plumbing or other utility failure at home.
Cousin to the Beach Episode and Public Bathhouse Episode. Every anime more than two episodes long will include at least one of these scenes.
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Anime & Manga
Done rather well in Gunbuster with some rather... brash detail to the girls
Mazinger Z: In chapter 8 of the original manga, Kouji and his friends spending a while on a hot springs resort in a mountain. That episode was noteworthy because Mazinger-Z did not show up and it had a very funny Kouji/Sayaka moment in the hot springs (with a double Luminescent Blush included).
The 11eyes OVA becomes one at the end. Although they are wearing bathing suits and its mixed bathing.
The 4th episode of 801 T.T.S. Airbats.
The second episode of Adventures of Kotetsu.
Played for laughs and lampshaded in the same chapter in Ai Kora when Hachibe invites all the girls along with him to an onsen he "won" tickets for (he really worked at a bunch of several part time jobs to earn the money to buy them, unsurprisingly.) simply so he can talk to Sakurako and figure out his feelings for her and vice versa, and also to peek at the girls of course.
Ai Yori Aoshi did this twice. In one of them, a little boy is frightened by Kaoru's terribly scarred back. This actually provides a Fridge Brilliance insight into his state of mind at this point. The fact that he didn't think of it shows that Kaoru's Dark and Troubled Past isn't so much on his mind anymore, thanks to his True Companions.
In the manga... how does North Italy find out that Japan's a lightweight in his pants? When they both take a bath in the same spring.
Also, Hungary and Taiwan bond over their love for hot springs via taking a trip to Lake Heviz, which is basically a huge Hungarian lake acting as such.
The third OVA for Blue Seed Beyond was a long contrived Hot Springs episode where the team had to stay in the onsen itself or else a bomb would detonate and kill them all. Of note, the main plot was already resolved in the second OVA, so this was apparently nothing more than an excuse for having female characters wear swimsuits (with Kusanagi in a speedo as a bonus).
Despite the mockery, they apparently couldn't help themselves; there's an omake-style chapter in the second manga (set during the first season, though) where Kiko gets her wish. Kirihara's there too, making the fact that it's pretty much just an excuse for fanservice even more blatant. Oh, and November 11 gets naked again.
Kogoro Mouri's Crowning Moment of Awesome happens in the Hot Springs episodes (a two-parter) of Detective Conan This time, both the victim *and* the murderer were dear friends of Kogoro, so he goes into a full It's Personal mode and we get to see that with a bit of nudging from Conan (who wanted to solve it at first but was moved by the other's utter determination to find the culprit and traitor), Kogoro *can* solve cases by himself.
Let's not forget the memorable scene where Ran and a rather shaken Conan return from the springs after they bathe together. Kogoro notices that Conan is unusually quiet, so when Ran tells him what they've been doing, he wonders out loud if he's dumbstruck after seeing Ran naked, and Conan has an epicNose Bleed at what the suggestion reminded him of.
The Digimon series has invoked one nearly every season.
Reversed somewhat in Digimon Xros Wars. Akari, the lone girl in the party at the time, was alone on her side of the rocks. When she noticed the villains of the week approaching, she peeked around the side to tell Taiki and Zenjirou. However, Zenjiro gets all freaked out and accuses her of peeping.
El Cazador de la Bruja has the main characters visit a hot spring in the middle of a Mexican desert. Lampshaded by having the characters actually say that it's common in Japan, and then by having one woman go in without a swimsuit on the grounds that that's how they do it in Japan.
Fruits Basket has an episode where Kyo, Tohru, Momiji and Yuki visit the hot springs resort owned by the Sohma family. Not as much fanservice for guys, but a fair amount for the female audience.
Full Metal Panic? Fumoffu. Episode 9, alternatively titled "The Hot Springs Episode", is a direct parody of the hot springs cliché (among others), and features three male characters trying to infiltrate a women's bath, fully protected by a steel wall between the male and female sides to prevent peeking, sentry guns and mines. Oh, and SEED PANTIES Mode, we can't forget about that.
There was also a bit of one in the original Full Metal Panic in the bath on the TdD.
One of the earlier episodes of Green Green has the school (which has just become co-ed) opening a hot spring, and focuses mainly on the boys trying to peek on the girls, ending with the boys all breaking the fence and spilling right on top of the nude girls, and later the main character searching underwater with one of his love interests for a key, to decide whether or not the guys get access to the springs. The springs appeared at times in later episodes.
Also of note is that Green Green is entirely a fanservice anime, meaning it shows the girls completely nude in the springs, especially in the final episode.
The first episode of Grenadier... and the second... and the fourth (both sento sequences)... and the eighth... the lady loved hot baths.
Mobile Suit Gundam: The 08th MS Team has a fairly contrived hot springs scene where the lead couple have to survive in a snowfield, and the hero creates an improvised hot spring to keep them warm by melting and heating the snow with his heavily-damaged robot'sbeam saber. It's even more contrived in Gundam SEED Destiny, where two of the female leads discuss things in an onsen on board a battleship.
In the Char's Deleted Affair manga, an artificial hot spring is constructed on Axis and the three women who are kinda interested in Char wind up trying it out, including a younger Haman Karn.
Hana Yori Dango has a couple of episodes that take place at an onsen, where Tsukushi passes out in the onsen and has to be rescued by Rui.
Iono the Fanatics has the cast spending approximately half of the huge first chapter in a hot spring.
Second season of Junjou Romantica has Usagi and Misaki vacationing at a onsen, where they unfortunately meet Usagi' father Akihiko.
In fact, a good third of pretty much all yaoi manga in existence will somehow have something to do with owning an onsen, taking a trip to an onsen, having sex in an onsen...
Love Hina is set in a girls' dormitory that used to be a hot springs resort; the girls living there use the spring, and the entire show can be considered a single continuous Hot Springs Episode (except, of course, for its several Beach Episodes).
Still an enormous amount of Fanservice, though. This is Ken Akamatsu we're talking about.
One early chapter earns points for confining a hot bath to a steel drum barely big enough for one person. There were two in it.
It came up again recently as a "break" between tense dramatic action sequences, in an apparent attempt to compensate for the slightly reduced Fanservice of the past year or two by turning it up past 11 to around fifteen or so, by heavy use of Skinship Grope.
"This week, we bring you a 100% in-the-nude chapter!"
And, indeed, no clothing is to be seen, other than a strategically placed towel or three.
It also inverts the standard "Guys try to sneak a peek at the girls side" idea. The only male who attempts this is Chamo, who's an ermine. (Negi and Kotaro decide that it isn't the time to worry about something like that.) We also see several of the (female) Adiane guards plotting to sneak over the males' side to see if "Nagi" is over there, but get interrupted when Chamo pops out of one of the drains.
Majin Tantei Nougami Neuro has an onsen episode in which Yako does not even get to enjoy the hot springs because one of Neuro's devilish creatures was set loose on the water.
The titular ship and setting of Martian Successor Nadesico had a simulated hot spring on board as a sauna area. Naturally, in one episode, local pervert Uribatake and his all-male engineering crew just happen to be searching for a Jovian infiltrator when most of the female cast is using the facilty. The hot springs room gets checked out quite thoroughly.
Naruto has had a few hot springs episodes, mostly played up for comic value.
In one episode, Naruto and friends use the hot springs as an excuse to see Kakashi's real face, only to find that he bathes with his mask on.
In the same episode, Sakura, who is presumably naked, is seen kneeling in the adjacent bath, though they chose to give her a Modesty Towel instead of Censor Steam.
In another, the cast stops by a resort on the way back from a long plot arc. Lovable Sex Maniac Jiraiya immediately heads for the springs to take advantage of all the Fanservice. The females, in a moment of clarity decide to spend their time in the casino rather than the water. While the plot unfolds we are treated to repeated cutaways of Jiraiya sharing the water with monkeys, Sumo Wrestlers, and elderly old women. By the time the young women show up, Jiraiya has been in the hot water so long he's passed out from heat stroke.
The post-Time Skip series Shippuuden continues in the same vein. When the new team stays at the onsen, the bathing scenes are just... bathing scenes, though we do see Naruto briefly considering peeking on Sakura. Then he's told what happened when Jiraiya tried it on Tsunade... and Sakura's her apprentice, with temper and strength to match. He decides he likes his ribs and internal organs where they are.
In one of the post-episode vignettes, Naruto takes the Sand siblings on a tour of Konoha, including the hot springs which the Sand doesn't have. Sand ninja fan service. Yay. :3
Even Neon Genesis Evangelion had one settled right after a mission. It featured Shinji bathing with Pen-Pen at one side, and Asuka and Misato bathing and talking at the other. Incidentally, it seems that every bath scene ends up with Pen-Pen seeing a bit more of Shinji than he probably would have wanted.
One Piece had a brief one at the end of Arabasta arc, although the fanservice there is strictly for females. In an amusing spoof, when Nami catches the guys peeking on her and Vivi, she gets rid of them by flashing them, causing them to pass out and fall back on their own side. She dubs this technique the "Happiness Punch" or "Paradise Punch", depending on the version.
There's a scene in a similar vein to a hot springs scene much later in the Amazon Lily arc where Luffy gets cleansed after being set on fire by the natives of the island. Despite this being Lady Land, only Marguerite, Luffy's eventual arc friend, is missing any clothing (Granted, the Amazons tend to wear little to begin with.), for no apparent reason other than law of fanservice and virtue of not being a Gonk.
Outlaw Star Episode 23, "Hot Springs Planet Tenrei" (as might be suspected from the title) dropped its Hot Springs Episode dead smack dab in the middle of the series' climactic plotarc. This episode was dropped from the dubbed airing in the U.S. simply because of how much nudity would have to be edited out of it. However, unlike typical fanservice episodes, there is a significant plot point in this one that may lead to confusion in following episodes, as Gene obtains four exceedingly rare caster shells; that's why he really went there in the first place. Another discontinuity involves Tobigera, one of the Anten Seven, who is named in the group's introduction, but only seen in this episode. The reason he's never seen afterward is just part of why this episode is also highly regarded for being funny as hell.
Patlabor had one sento episode. Which was a subversion in several ways. Firstly, it followed the male members into the bath, to locate a mad-bomber with "three moles in the left armpit". Fanservice was not a major point (though ye olde black dots got a workout), the guys never even tried to sneak a peek at the ladies' side. Ironic since the bomber was a woman.
A later Patlabor OAV "Versus" could be considered a Hot Springs Episode. Although it would be better called a Ryokan (Inn) episode. The characters are only seen coming back from the springs and the whole episode is about a contest/rivalry between Kanuka Clancy (back off the bus, on vacation) and her replacement Takeo Kumagami. Which starts with trivia questions, escalates through drunken tongue-twisters, and ends up with seeing who can throw Ohta out a window better. The only character we see in a spring is Captain Gotoh (oh, and Ohta, but he?s wearing a robe).
The Patlabor manga has several instances where characters are taking a bath or a shower. One has Asuma and Ohta naked except for towels covering their you-know-whats. That is, until one takes offence at what the other says, and they start to fight. Then the towels are replaced by steam...
Episode 4 of the original OAV ("The Tragedy of L") features Captain Gotoh sending the SV2 pilots to a rural training camp, which includes a communal bath. (Amusingly, we get to see that Kanuka Clancy keeps a handgun with her even while she's in the bath.) During the bath scene, the water on the men's side suddenly turns red, scaring them, and from that point onward the episode becomes a ghost story. Turns out it was just a hoax plotted by the two captains, to teach the pilots a lesson about gun safety.
Ranma ½ has both variations; the bathhouse more often than the springs (the bath seemed to break every other week and this was sometimes just used to start the story). One time the hot spring is large and circular, with a rock outcropping providing a wall through the middle, separating it into men's and women's sides. The men try to peek around one end, and the women try to peek around the other end at the same time. They don't see anything, so they dismiss the sounds they heard as delusions.
Inu Yasha has one every dozen or so episodes, particularly early on; the villagers find Kagome odd for her love of "purifying herself" in springs, but she eventually turns Sango on to hot spring bathing. This results in one of the series' Running Gags - Celibate Hero Inuyasha charging to the rescue after Kagome screams (sometimes even for him) only to end up an Accidental Pervert and subjected to a "sit" command. Once Chivalrous Pervert Miroku joins the band, this escalates to Inuyasha's efforts to deter his Girl Watching not only getting the monk clonked in the head by Sango's boomerang (even if she's not CARRYING it), but Kagome "sitting" Inuyasha just for being present.
Somewhat subverted on Read or Die: The TV. They all go to the hot springs, but none of the main characters remove so much as their jackets. The one 'guest star' who does take off her clothes ends up brutally murdered. In the spring. And then her bodyguard activates an ancient device that destroys the springs entirely in a massive explosion. Hmm. Maybe more than 'somewhat' subverted.
Magical Pokaan features one in which the girls have an unfortunate encounter with tanukis.
The Rurouni Kenshin anime has not one, but two onsen episodes. Both involving characters accidentally seeing each other naked.
Saber Marionette J had a Hot Springs episode, giving a chance for gay Hanagata to rub up on Otaru wearing nothing but a very small towel.
Saiyuki: Reload Gunlock has an onsen scene: Sha Gojyo is draped with a towel; Cho Hakkai is just an outline in the water (but what an outline!!)
Sayonara, Zetsubou-Sensei has a hilarious episode in which the gang decides to detox at a hot springs facility. Fanservice is mocked, the class of highly problematic students end up losing their quirks and becoming normal... leading Itoshiki to re-tox them with highly caloric food. He then gets pushed onto the de-toxing spring himself, then dramatically disappears. He was all toxins.
For bonus points, count how many times Kagero Usui can be seen on the girls side, mostly unnoticed by even the viewers. At one point, there are several Kagero Usuis all over the fence.
Sukisho the last episode. Taking into consideration that it is more or less filler it is not surprising that fanservice should abound.
Just about every episode in all the Tenchi Muyo! series have at least one scene in an onsen. The first TV series has Washu build one into a Pocket Dimension attached to the bathroom - a woman opens the door, and enters an opulent onsen with steaming water fountains. A man opening the door just enters the normal bathroom. Ryoko builds the onsen in Tenchi Muyo! Ryo-Ohki. Lacking Washu's ubertech (or at least, lacking Washu's ability to invent new ubertech on a moment's notice), she just built a Floating Island behind the house and put the onsen there.
The sixth episode of Tengen Toppa Gurren Lagann had the main characters directed to a hot spring by an old man. Kamina and Simon needed the concept explained to them, as both had grown up underground. The spring turned out to be not only a trap set by the Beastmen, but a Ganmen in disguise. Also, there was a wardrobe malfunction. Followed by an explosion.
The Wallflower had an episode set in a hot spring, which featured a murder mystery, Sunako generally creeping out everyone staying at their hotel, and a truly awe-inspiring game of ping pong.
This Ugly Yet Beautiful World had a hot springs scene where ALL of the series' female characters participated. Granted, nudity in This Ugly Yet Beautiful World is more plentiful than Ho Yay in Revolutionary Girl Utena, but the hot springs scene shows almost full nudity for all the characters, including the one nude shot of the female preteen character. Then the very next episode was a Beach Episode...
Was there a Hot Spring in an Episode of Voltron (or was it Macross)?. Well not so much a Hot Springs scene in Voltron, more like a what happens when you combine the boys, Allura in a bikini and her doing a high dive into the lake that surrounds the castle. She and one half of her swimsuit end up surfacing in one location in the lake... the other half of her swimsuit shows up elsewhere.
Possibly the only time this has ever happened without any intended fanservice is on Yu-Gi-Oh! GX, when Judai/Jaden and allies visited the school's onsen.
Gravion, naturally, has a Hot Springs Resort episode, and naturally (being Gravion), it still ends with a Giant Robot battle.
ZokuNatsume Yuujinchou pulls this in one episode. Turns out that Natsume was invited along specifically because there was a Book of Friends bound Youkai loose in the resort.
Episode 19 of The Galaxy Railways has the members of Sirius, Spica, and Vega platoons vacationing at a Hot Springs ouned by Captain Murase's parents.
In Tantei Gakuen Q, two multiple murders happen in hot springs. Only the second one had fanservicy scenes: Kintarou first attempts to forcibly strip Ryuu off his modesty towel, then he and Kyuu try to peep on a Sexy Secretary who's bathing at the other side but end up seeing Megu nude instead. And the latter incident ends up being one of the clues to catch the Serial Killer... who happened to be said Sexy Secretary.
Episode 5 of Saki has the characters going to one for their Mahjong training camp.
As well as the ending episodes of season 1, which invited every member of the mahjong team to join them before the next round started up, notably leaving behind all of the male characters.
Shakugan no Shana had the Hot Springs OVA. There was also a pool episode, but it probably doesn't count (although that one has high doses of fanservice).
Pokémon Special had two incidents involving hot springs. One in the G/S/C arc was used by Sabrina and, later, Red. This was not due to fanservice; they were both frostbitten by Lorelei in the previous arc and needed to recover.
And then there's the Lavaridge Gym battle between Sapphire and Flannery — yes, a Gym battle in a hot spring. Sapphire wanted to do it right then and there, however; Flannery and Mack used their strongest fire powers to slow down Mt. Chimney's complete death after Team Aqua killed the volcano, and it was now or never for the hot springs.
The Pokemon anime had a Hot-springs Episode during the Cinnibar Island arc, where Ash and Brock are on the boy's side and Misty - wrapped in a Modesty Towel is on the girl's side... until Togepi demolishes the wall between them by activating the secret door to Blaine's gym.
Lampshaded and subverted in Hayate the Combat Butler Ch. 117. After the characters spent about 5 chapters trying to get to a special hot springs, the first thing Hayate and Maria do is to relax... in massage chairs. Another character even exclaims, "Why hasn't the bath scene appeared yet?!"
Notably though, the main character isn't interested in peeking, unashamedly entering one of the springs when his (female) master has the effect of getting drunk. It seems the 'girls' are more likely to peek at each other in this story.
And Ai confesses at a hidden hot spring while nude.
Played with in Keroro Gunsou, where one of Momoka's poorly thought-out plans to get closer with Fuyuki involves letting him win a set of four tickets to an onsen, and hoping he takes her as a guest. Most of the action involves Momoka fighting with Natsumi's friend/admirer Koyuki over the extra ticket, to the point where they don't actually get to the hot springs until the very end of the episode.
The Weatherman Is My Lover features an extra where the cast and crew of Chou Hayaoki TV have what amounts to basically a school field trip to an onsen.
Samurai Champloo has a few instances where the characters visit hot springs, including one where Jin finds himself in one along with an Ambiguously Gay samurai who intends to kill him.
Hell Girl had one where Hell hotline was instrumental in the building and destruction of a hot spring.
Transformers Cybertron manages to make use of this trope in one episode. Notable for it taking place in Alaska.
One in Fairy Tail with Lucy and Erza bathing together during the Loke arc.
There is another one in chapter 261 with Lucy, Erza, Wendy, Levy and Juvia. Natsu, Gray, Jet and Droy try to do some Outdoor Bath Peeping. This was initially omitted from the TV anime but was instead made as the fourth OVA where the fanservice was increased.
In Katekyo Hitman Reborn! they play the cheap version in when Dino's pet turtle destroys their bath tub. (and most of the house)
Koibana Onsen is about three sisters running an onsen. Twice, they take a vacation to visit an.... onsen!
Anpanman has the character Onsen-kun, who actually makes hot springs. Every starring episode he gets will be a hot springs episode. The show is meant for a young audience, so no Fanservice is intended.
This happens in Ginga Densetsu Weed, where Tesshin and his pack lead Weed and the soldiers to hot springs to recover from their wounds and enjoy themselves. While there, Jerome (who went missing the episode before) appears there with a casual "Yo", and Tesshin gives Weed a moving speech Gin told him to give to him.
Not only that, but Weed also learns the Battouga and uses it on a trio of boars ruining the good time.
One Piece: Parallel Works had Yuki-Rin, Kazuma, Heathcliffe, and Aki going to one during the Five Days of Fun Arc. It didn't really have any Fanservice, but Heathcliffe's backstory and Yuki-Rin's amnesia were discussed among the Capricorns.
The Berserk fanfic "Three Nights" pulls this off with Guts, Casca, Pippin and Judeau finding a natural hot spring on their way to Wyndham to rescue Griffith. After Pippin and Judeau bathe off-screen, Casca aggressively but cleverly convinces Guts to take a bath by luring him to the hot spring - with her as the bait. The tone of the fic avoids this scene from turning into a lemon, but Guts and Casca do share a very intimate moment with each other.
In the 1954 film The Bridges at Toko-Ri, Harry Brubaker (William Holden), an American fighter pilot stationed in Japan goes to a public bath with his wife Nancy (Grace Kelly) and their kids. No nudity, though, since it was 1954.
Well, they seem to be nude, and act very shy when a Japanese family enters and disrobes for the bath next to them. Later, they relax. Pretty daring for 1954.
In The Yakuza, Tanner enlists the aid of his old army buddy, Harry Kilmer, to go to Japan and rescue his child. Or, so it seems. There's a bit of tattooed knife violence and implied nudity in the bath house.
Memoirs of a Geisha, has the titular character take some high ranked americans militars to a onsen resort.
Live Action TV
Star Trek: The Next Generation, of all things, had one. Troi, Troi's mom, Worf and his boy Alexander all hopped into mud baths on the holodeck a couple of times.
A rare Western example: I, Jedi has a scene where Luke and his inaugural class go swimming in a grotto. Corran also discovers that Halcyons have to obey the laws of thermodynamics, as he absorbs the heat and then redirects it into psychokinesis.
One of the inns in Tales of Symphonia is also a hot spring, and going there with Zelos in the party after a certain plot event results in a cutscene where Zelos tries to spy on the female party members while they're bathing. Lloyd shows up and chastises him, but the ladies hear him and Zelos runs off, leaving Lloyd to take the blame. The game even gives Lloyd the title of "Peeping Tom".
The same kind of situation happens in Dawn of the New World if the party goes to the hot spring at one point and have the ladies go in. Zelos tries to spy on the females (while Emil and Lloyd give him crap for it), and Lloyd gets blamed again as he's the only one didn't run. When the males take their turn, Marta tricked everyone but Emil into not coming in so she could spend time with him. When Emil finds out he ends ups calling everyone else attention. Naturally, everyone but Lloyd decide that Emil must have planned this even though both claim otherwise (and them all knowing how openly affectionate Marta is towards Emil), and Emil also gets the title "Peeping Tom".
The original bathing scenes in Tales of Phantasia had peeping on both sides: Chester on the male side, and Arche on the female. Chester also gained the title of Muscleman, Mint the title "Boin~chan". Tales Of Eternia had no wall-watching, although Reid did have to smuggle Chat into the men's dressing room to get one of her skills, which leads to a rather unfortunate scene between her and Keele. Also on a seperate occasion, Meredy decided to bath near the man's side instead, and Hilarity Ensues.
They're at it again in Tales of Vesperia. There are three different scenes. To get the first two, you need to pay 300,000 gald for the first and 600,000 gald for the second. During the first two Raven keeps trying to peak at the woman's side of the spring, with little success. The third visit is free, During the third, the party winds up working there and at the end they all get costume titles.
In the PS3 version, there were a few scenes added for the hot springs to accommodate Flynn and Patty being in the party.
Actually, the second time Raven manages to see Judith completely nude, and coerces Karol into being his accomplice. Judith appears to be aware of this, but utterly nonchalant. In the the final scene, after returning with the new costumes, Raven and Rita have a shouting match through the partition.
Riviera: The Promised Land and Yggdra Union both feature a hidden "hot springs scene" where the games' female characters appear naked. Bafflingly, the American version of both games' hot springs scenes actually shows MORE than the Japanese version.
One of the jokes about Riviera is that the only way to tell gender for some NPCs is whether or not they appear in the bathing scenes; they are female if they appear in one, and every female appears in one.
Lunar The Silver Star features sex segregated springs, with the male-only one much easier to reach than the female-only one. This is also a bit of a Guide Dang It since you (naturally) need to bring soap. It's only available for a very short time at the very beginning of the game. Long before there is any hint either spring exists.
The sequel Lunar Eternal Blue features baths instead. The clueless Lucia walks into the male bath as well, much to Hiro's surprise - and Ruby's chagrin.
Part of the Mt. Gagazet sequence in Final Fantasy X-2. (But only as a reward if the player approaches the area in a particular, stealthy way. If they approach the Fem-Goons directly, then the scene is skipped.)
Later on, as the Gullwings install cameras all over Spira, they get to watch nearly the entire cast of NPCs show up at the hot springs at various times. Subverted as all the NPCs keep all their clothes on (most probably due to budgeting). The PC sequence does provide briefer outfits for the three girls, although in Rikku's case it can be argued she ends up wearing more than she usually does.
Just outside the spring, an elderly man is warming himself in some heated sand. Talk to him, and he'll remark on how good the sand feels—until a Pokemon bites him in the butt. Perhaps dampening the comedy: the primary sand-dwelling Pokemon around there is Trapinch, a giant antlion larva with bear trap-like jaws.
There's a hot spring side-quest in Luminous Arc 2. And another in Luminous Arc 3. The reward for undertaking the quest is permission for your party to use the hot springs for a while.
Partially averted in Ōkami, in which Ameterasu tries to go to a hot spring to relax, but the spring isn't running. You have to go on a mini quest in which you dig underground to get it flowing again.
In Breath of Fire IV there is a scene that was removed from the international releases of the game, featuring Nina and Ursula visiting a hot spring during the camping scene in the Tidal Flats. They ask Ryu to stand guard, Nina makes a comment along the lines of "Wow, I had no idea you were so big!" he gets curious, Hilarity Ensues, unfortunately the clip seems to have gone missing.
Stage three of Touhou's second fighter Hisoutensoku starts at the "The Geyser Underground Center Entrance" which Marisa describes as a hot spring. Considering this hot spring is directly a product of Touhou 11 Subterranean Animism, and the next boss fight after Marisa takes place in the nuclear reactor powering the hot spring, this scene does not come out of nowhere at all. And no Fanservice.
Hot springs are Mother 3's primary equivalent to the Trauma Inn. There's even a little sign by most of them with a haiku on it detailing the location (though the syllable count is rarely correct). It's played with on Tanetane Island, however; while the human protagonists are hallucinating after eating some strange mushrooms, what appears to be a lavishly decorated hot spring is actually a pool of toxic-looking sludge, which Boney will refuse to follow you into (should you choose to do so). The actual hot spring is right next to it, though inaccessible until you've 'sobered up'.
As for a Hot Spring Episode, there's Lucas's meeting with Ionia in a hot spring, wherein Lucas learns PSI from Ionia... somehow.
Present in Bleach: The Third Phantom. If you chose to play as a girl, you get more scenes than if you chose the boy.
Present In Asura's Wrath, with Asura and His old master being served by Hand maidens.
Each game in the Suikoden series has an onsen built in the player's headquarters, and various optional scenes can be viewed by bringing the right combinations of characters.
In Muramasa: The Demon Blade, the male protagonist Kisuke and the female protagonist Momohime can have a series of humorous encounters with each other at a hot-spring.
On Chapter 16 in the Japan route of Super Robot Wars UX, the teams go into hot springs in the hidden ninja village. Sakura asks Sonshouku from SD Gundam Sangokuden if she will rust or not. No official art.
In one El Goonish Shive strip, the characters recapping the plot to each other is covered up by scenes from the "Lucky Bunny Bounty Show" — both of which involve the male character accidentally walking in on the female character in an onsen.
In the creators' commentary for A Miracle of Science, a Hot Springs Strip was repeatedly referenced, originally in an April Fools joke.
Red String, which faithfully uses all the tropes of shojo manga, features an entire chapter about the cast spending a few days at an onsen. One of the male characters sees more of the anatomy of three Hot Shounen Moms than he intended to.
The Animesque animated series Totally Spies! has an episode where the girls take a vacation to a ski resort and relax in a hot tub. It's not long before a snow-and-ice gimmick villain shows up to ruin everything.
The Smurfs episode "Smurfs On Wheels" is basically three Smurfs on a hiking trip versus a group of Smurfs riding in Handy's new Smurf Wagon in order to get to Dreamy Steamy Springs. Naturally the hiking Smurfs get there first while the wagon-riding Smurfs only get there by way of Brainy's driving only for the vehicle to end up heading for the waterfall.