"I was driving in downtown Atlantis My barracuda was in the shop So I was in a rented stingray and it was overheating"
— Kip Addotta, "Wet Dream"
Living underwater sounds like it would be so cool, doesn't it? Actually, in fiction, it isn't that big a deal because life at the bottom of the ocean is conducted impossibly similarly to life on land. Whether your characters are mermaids, Fish People, or talking, sentient fish and other sea creatures, you'll find their underwater lifestyles have a lot in common with humans' above-land lifestyle. Most Writers Are Human, and they must want to give the viewers or readers a portrayal they are familiar with.
Buildings designed by and for mermaids but built with things like stairs (even though they have no legs) and upper story windows (which would function the same as a patio door), and furnished with chairs or beds, even though buoyancy means you would float over them.
Bonus points if characters like mermaids ignore this trope and swim in and out of upper story windows anyway.
Coral equals plants and grows everywhere.
Characters move in two-dimensions, walking along the sea floor or swimming very near it, despite that even we humans can move in three-dimensions in a swimming pool.
Water is always crystal clear, and mud on the sea floor stays on the sea floor rather than dispersing as clouds of silt at the tiniest disturbance.
The effects of water pressure and temperature are non-existent.
Sound being transmitted as clearly (for human ears) as through air. The direction of sounds will be easy to pinpoint.
If it's daytime, there will be sunlight. This can sometimes be acceptable depending on the elevation, but since most stories are on the bottom of the ocean, this makes no sense.
Long hair still hangs down instead of getting tangled around your head.
Alternatively, long hair will act like it would underwater, but it will just sway around in artistically convenient ways, never tangling or getting in the way.
Characters can pour and drink cups of other liquids. (This is possible if the liquid has a higher density than and immiscible with water, but it never looks like it does in fiction.)
If you really want to stretch it, things can burn. With fire. Yes, underwater. (This is possible with proper fuel, which usually isn't used in this case. This is usually Lampshaded.)
Or stretching it to extremes, there are rivers and lakes. Underwater. Which hardly-if-at-all resemble brine pools or the real phenomenon of underwater lakes and almost certainly were not designed with those in mind.
Characters (who can swim) are worried about falling into trenches and off cliffs.
All forms of combat can be executed as if on dry land. Projectiles, blades and bludgeons inflict the expected amount of damage despite having to pass through a medium 784 times as dense as air.
Sinking objects descend at the same speed they would if they were falling through air.
If there are talking animals, creatures like whales, seals, and crocodiles can still talk underwater despite not being able to breathe it.
Electronic or paper devices work just as well as on land with no adverse effects.
The problems with viewers being able to understand the characters can sometimes be handwaved with the Literary Agent Hypothesis.
In addition, the setting for underwater fantasy is almost always "under the sea," not "under Lake Michigan" or "under the Amazon River." After all, we know more about the surface of the moon than the bottom of the ocean, which gives writers far more leeway to bend the laws of physics (or so the evidence would indicate) than in a freshwater world. Well, apparently, there is hardly difference between salt water and fresh water anyway. Good thing, too, since that taste constantly in your mouth would probably get annoying faster than permanently wrinkled skin and burning eyes.
In short, replacing air with water requires far more changes to the human anatomy than simply replacing legs with a fishtail and far more research about the laws of physics and biology than most writers bother to do. Fortunately, as mentioned above, the concept is saved by the Rule of Cool and by Acceptable Breaks from Reality. Nobody is going to watch an ocean movie, only to strain their eyes and ears to see or hear anything taking place.
Compare Water Is Dry, Space Is an Ocean, Sand Is Water, Atlantis Is Boring, and Walk, Don't Swim.
Contrast The Sky Is an Ocean.
Not to be confused with Super Not-Drowning Skills, when characters are given an unexplained ability to survive underwater for an infinite time.
Digimon Tamers did this in "Shibumi Speaks". As long as they believe they won't drown/get wet/etc, it won't happen to them. Justified in that the Digital World functions under vastly different rules than the real one.
Nagi no Asukara has this trope full-scale. Underwater is exactly like above water, except for fishes (who looks more like hovering than swimming) and the occasionnal swimming (which looks rather more like a long jump). There is no buoyancy, no water resistance, no distorted sounds, no light dampening and you can even cook and drink without a problem.
In Neon Genesis Evangelion, LCL has the consistency (and, presumably, composition) of amniotic fluid, and characters breathe it while sitting in the Entry Plugs. Fair enough, but they also speak and yell without any sort of difficulty or distortion. Sweat, blood, and tears also behave as though LCL had air-like density and solvency. There have been attemptsto explain away the unimpeded-speech issue, but the tears that fall freely on a character's lap remain inexplicable.
A very odd example in Saint Seiya. The Seven Pillars of Poseidon's Sanctuary are connected to, and hold up, the Seven Seas, so the temple itself —at the bottom of the ocean, mind— is a dry land above which the seas hang like a canopy. As a show of force, Seiya was once punched upwards so hard by Seahorse Baian that he crashed into the "ceiling" (namely, the bottom of the North Pacific Ocean,) was pushed up all the way to the surface by the strength of the blow, and then sank all the way back down, unimpeded, and fell right back into the Sanctuary. But despite the entire depth of the ocean hanging above the temple, sunlight is as abundant there as though they were fighting in an above-water plaza.
Marine Boy was all over the place with this trope. The Ocean Patrol craft certainly moved in 3D, and required engines to do so. While underwater, the characters never walked, and the resident mermaid had, at times, to cope with not having legs, though we never really saw her much away from the humans. On the other hand, the Non-Human Sidekick (a dolphin) never needed to breathe (and it's doubtful that he could chew the "oxygen gum" that the eponymous hero used); the hero's uniform had no visor or goggles, yet he had no difficulties seeing or talking underwater — which was generally crystal-clear; and in the most outrageous use of this trope, his sole weapon was a folding boomerang, which he threw at everything from bad guys to sea monsters to full-sized submarines! Being an "electro-boomerang", it zapped them all (and frequently more than one in a single "flight"), often causing mechanical enemies to blow up.
Inverted in an episode of Doraemon, where the gadget-of-the-week permitted the protagonists to treat air as water, for recreational purposes.
In an episode of Pokémon, they have a battle underwater. Misty scolds Ash for forgetting that you can treat underwater like the sky when Ash gets confused after their opponent takes advantage of the surroundings to attack in 3 dimensions.
This was, however, averted in the two most recent Aquaman ongoing series. It was even discussed at one point, with narration talking about how your typical Atlantean thinks in three dimensions. Indeed, modern writers play clever subversions of this trope in order to remind the reader how powerful Aquaman really is. If Aquaman is able to show off Super Strength and treat water as if it were air, imagine how much stronger he can be when he actually is in the air.
Fish Police zig-zags the hell out of this trope, often lampshading it as well. A notable example is Inspector Gill asking, "If we're under water, how come the beer stays in the mug?"
One of the crew, on the bonus DVD or making of or commentaries or some such, mentions that they actually originally made the water "too realistic," and had to go back and make the effects more "cartoony."
One of the most brilliant moments in the movie is when Marlin is shouting after Nemo, who was fishnapped by the dentist. He yells for a while, then goes down, takes a "breath" of water, and goes up and yells after him some more.
In Pinocchio, Pinocchio and Jiminy Cricket go looking for Gepetto underwater. The fact that Pinoch doesn't drown may be handwaved away by his being made of wood (though later he DOES drown...), but what about Jiminy? Well, he was in a bubble... briefly, until it filled up with water. Probably gave many physicists headaches.
Disney's The Little Mermaid film and its TV adaptation particularly suffer the problems of architecture (although granted, there were some fauna that primarily walk on the Ocean Floor, so the inclusion of something like "stairs" is excusable), coral=plants, and burning fire (usually blasts from Triton's trident), although long hair moved slower and tended to float (but still never gets in the way aside from surfacing). On the other hand, the animators did attempt to replicate the physics of water as realistically as possible in the original film.
Was always bugged by the inclusion of so many freshwater fish in "Under The Sea": trout, most chub, most carp, quite a lot of bass... and what's up with the newt (a freshwater salamander)...?
They needed something that rhymed with lute
Ariel specifically mentions that she doesn't know what fire is in one of her songs.
In the Prequel, Ariel sees something she wants to check out from her bedroom window... and instead of swimming out the window to check it out, she goes all the way downstairs through the castle to get outside. partially Justified in that she was sneaking out and needed to stay hidden.
Sebastian has the line "let's take you home and get you something warm to drink." HOW?
Shark Tale lampshades this when the race seahorse that Oscar bet on trips right before the finish line:
Oscar: He trips underwater? Now who in the halibut trips underwater? And, by the way... on what?
The answer is that the seahorses use an odd variation of the Walk, Don't Swim trope to "sprint". Their tails push off the ground, low enough that if there was a rock in the seahorse's way (there wasn't), he would've tripped on it.
Films — Live-Action
Sky Captain and the World of Tomorrow includes a scene where airplanes fly underwater. The airplane's control surfaces allow it to function almost identically to how it would fly through the air.
Disney's Bedknobs and Broomsticks, during the animated sequence after the bed and its passengers crash in the Island of Naboombu's lagoon and sink to the bottom.
In G.I. Joe: The Rise of Cobra, while not obvious, the effects of particles in the underwater fight scene moved very much like they were in air, not accounting for the difference in density, especially in cold water. In fact, the speed at which it moved was comparable to that in air.
In "The Magic Voyage of Sinbad" the title character throws himself into the sea to appease Neptune. On MST3K they even ask "Why isn't it wet underwater?" and when a pigeon with a message reaches him in Neptune's kingdom they ask "How does that work?"
Voyage of the Dawn Treader addresses this briefly when Lucy sees mermaids through the preternaturally clear sea-water. Among other things, C. S. Lewis compares the deep water to dangerous mountains, and the shallows to sunny, habitable valleys. In addition, even though mermaids are usually depicted in media as being able to poke their heads above water and converse and breathe in air, Drinian explains that these merpeople cannot come up and examine the Dawn Treader or talk with them because... they cannot breathe air!
In fact, when Lucy sees them, she expects them to be able to surface, because her coronation apparently featured singing mermaids that could breathe air. Drinian explains that those mermaids must have been a different sort.
James Blish's novella Surface Tension averts this trope very nicely. Blish's microscopic water-dwellers live in a "universe" with three "surfaces": the bottom, where the water ends; the "sky", the top of the water, which (as the title suggests) they cannot penetrate; and between these, the thermocline, the division between the sunwarmed upper layers and the cold deeps.
The idea of microscopic brains still having enough neurons available for humanlike intelligence is a separate trope.
More an example of Science Marches On, since the microscopic people were genetically engineered to have cells the size of viruses, and when the story was written we knew very little about the structure of viruses and not a lot about the structure of cells.
In The Chronicles of Amber, the water in the undersea city of Rebma works like this, but the trope is justified as it's explicitly a magical effect, and people are able to breathe the water within the city as well. In fact, if you fall off the underwater stairway leading to the city, you re-enter 'normal' deep sea conditions and are crushed to death.
"Why are the waters in Rebma so different from the waters elsewhere?" I asked.
Averted in Hal Clement's books, since they are hard SF. In one case, a colony of humans is established on the ocean floor, using geothermal power to provide light and a specially-made oxygen-carrying dive fluid in place of air. But since the dive fluid is denser than water, the humans have to wear weights if they want to stay on the bottom or even have neutral buoyancy (their bones were denser than the fluid and their lungs were filled with it, but the rest of their bodies were less dense and the net effect was a slight positive bouyancy). They sleep tied to the ceilings of their buildings.
Played with in Close to Critical. The planet Tenebra has atmospheric conditions close to the critical point of water. This leads to some truly bizarre effects like large blobs of water hovering in the air, and people lighting fires to drive water away at night.
In the children's fantasy novel Lundon's Bridge and the Three Keys, common examples of this trope appear (some, such as humans and insects breathing underwater and having no problems with pressure, are handwaved as the result of magic) — and then there's the crisis that kicks things off. The world's oceans are gradually being overtaken by "The Decayed Sea", polluted waters that mutate plants into carnivorous monsters. Decayed Sea areas are the aquatic equivalent of "forbidden forests" in land-based fantasy works; one can enter (more often, be pulled into) and exit them, and the monsters cannot survive in the clean water beyond them. This shows a gross misunderstanding of how water and contaminants work, but correcting it would require a thorough rewrite of the plot. (For more specifics, read the opening pages of the book via the "Look Inside" feature at Amazon.com.)
Mostly averted in Redwall: since the main characters are all mammals, there are very few underwater sequences, and even those involve otters.
seaQuest DSV takes place on a hyper-advanced submarine in the near future. They almost always treat water as air (they even have mini-subs that behave as jet fighters).
This happens all the time in the U.S. newspaper comic strip Sherman's Lagoon.
The Basic D&D supplement The Sea Peoples averts this trope, mentioning such issues as light levels, water clarity, and triton architects' channeling water currents through homes so that oxygen-depleted water is carried away efficiently. (The last chamber that such disposal-currents pass through is even designated as a latrine.)
Later editions of Dungeons & Dragons have pages of rules detailing precisely how life underwater is not like life on land (and spells to remove some of these differences).
In AD&D1E this trope could be invoked by means of the Airy Water spell, which caused an area of water to be simultaneously treated as air for air-breathing creatures and as water for aquatic creatures.
Star Fox Command doesn't even attempt to distinguish between normal and underwater levels. All ships are perfectly operable and there is always enough light. Oddly enough, water surfaces are considered solid obstacles in normal levels.
Star Fox 64, on the other hand, did a pretty good job at averting this in its only underwater level, Aquas. Fox operates a submarine, it's dark (although the script implies this is at least partially due to pollution caused by Andross' bioweapon) and the enemies are fitting for the setting. Why Andross would deploy a bioweapon down there is another matter...
the official strategy guide implies that the "bioweapon" in question, Baccoon, was actually an entity from several millenia prior whose envy of the surface dwellers resulted in him sending explosive starfish to blow up the icecaps and melt them resulting in Aquas becoming the 100% ocean planet that it is in the game, or at least was named after that entity.
X-Com supplement Terror of the Deep features humans fighting aliens in an underwater world. Unfortunately, the game system was directly adapted from the original with no changes, so the characters are able to do ridiculous things like throwing grenades underwater.
There are also surface missions. The only difference is the type of aliens you encounter and the fact that your "flying" suits can't fly in air. All the alien (and reverse-engineered) sonic weapons still work exactly the same, despite the fact that their firepower should be vastly reduced in air.
Final Fantasy X: Blitzball is a cross between hockey and rugby (or soccer, rugby and diving, depending on where you live) played by two teams of 6 (five players plus a goalie) played entirely underwater in five minute rounds. It is stated that the characters have learned to hold their breath while doing incredibly strenuous activity for this amount of time to become players. The fact that the water itself is specifically designed to help improve breathing duration (apparently due to pyreflies saturating the water) also helps.
8-Bit Theater parodies this with "Drownball", a more realistic version of Blitzball. Fighter proves himself decidedly amateur when, while using the daring strategy of wearing full armor while underwater, he loses because he "did a thing where I didn't drown." In a perfectly logical turn of events, however, the title of champion defaults to him anyway because he's the sole survivor of the match.
There's also the fact that all of the players move exclusively in two dimensions, as though they were playing on land, despite the fact that the "playing field" is literally a large sphere of water. Although earlier cutscenes show the players making use of the entire volume of water, with Tidus making a spectacular leap outside the sphere at one point, the only time Blitzball gameplay makes use of the third dimension is during Tidus' Jecht Shot where he swims up to make the kick.
They can throw a ball underwater as if there was no water in the first place. Especially notable since Blitzballs are shown to be light enough to float on the surface of the water, and have the dynamics of soccer balls when handled in dry land.
Final Fantasy VI averts this trope slightly, as a diving helmet is needed to access the ocean path. Then again, only one is needed (or found), and no explanation is given of its functionality without other supporting equipment.
In Final Fantasy VII you can go underwater in a submarine. Fair enough, except that you can fight Emerald Weapon while standing on the bottom of the sea, with no lighting or pressure-related problems. The air isn't breathable without the Underwater Materia, but the characters apparently can hold their breath for twenty whole minutes. (Eat your heart out, Guybrush Threepwood!)
Let us not forget that Cid Highwind will always fight with a lit cigarette in his mouth, even underwater.
While the previous game (and other levels in the same game) avert this, the ocean section in Alice: Madness Returns is something like this. Sea creatures can swim, but Alice walks and breathes as she would normally. Then again it is Wonderland.
Super Mario Bros. Mario could actually throw fireballs underwater, light fires underwater and in a Mario & Luigi game, Luigi could use electricity/thunder powers underwater. Not to mention the fact both Mario and Luigi, as well as pretty much any enemy in the 2D games could breathe underwater forever, or the underwater Chargin' Chucks (football players) who can whistle underwater. Or Wario's underwater working jet pack hat and flamethrower. Somewhat averted as movement underwater tends to be slower and more sluggish than normal.
Somewhat averted in Super Mario 64 in that Mario will drown if left underwater for too long, but the gauge which measures how long he has before drowning can be refilled if he collects coins underwater.
It also doesn't matter where his head is - so long as even just part of his shoe is sticking out of the water, he can breathe just fine.
Generally averted in The Elder Scrolls games, visibility is quite limited in water with light and night vision spells having little effect. Torches are extinguished when entering water. Pressure/depth issues are ignored.
It's averted in the material realm once Raziel learns how to swim/resistant to water. This even carries over to the sequels which is odd because all his other skills sort of disappeared.
Played straight in the Atari Lynx video game Turbo Sub. The player's aquatic jet maneuvers equally well under the ocean as it does over it; underwater enemies might look different, but their attacks and movement are the same as their airborne counterparts.
Averted in the first Mega Man X. The flamethrower weapon Fire Wave acquired from Flame Mammoth is completely useless underwater, barely distorting the water with the heat from X's Arm Cannon and causing a few harmless bubbles. Even the powered up version sends out a tiny wisp of flame that is extinguished the instant it leaves X's cannon.
Similarly, Burner Man's Wave Burner from Mega Man & Bass can't create flames underwater, but it will create a jet of hot water that can push enemies away.
Also averted in Mega Man X2, where if one uses Speed Burner underwater, only two pods come out (supposedly what carries the flame).
And in Mega Man X3, the Acid Burst dissolves in the water (only a few bubbles come out), making it useless.
Also averted (Though playerswish it wasn't) in the Ruminoa City ruin of Megaman Legends 2. For the underwater parts Megaman needs a "rebreather" to let him breathe and hydro skates to move around because his jet skates don't work underwater. He moves very painfully slow with seriously impaired jumping, and his armor is too heavy to allow him to swim.
In Metroid Prime, Samus' Plasma Beam functions perfectly fine underwater. This sort of gets a Hand Wave because she's firing superheated plasma as opposed to ordinary fire, but it still doesn't explain how organic enemies can catch fire when completely submerged.
Her flamethrower Charge Combo won't work underwater, though.
World of Warcraft: Vashj'ir. Full stop. While it avoids some of the more egregious aspects of this trope — for example, you can swim in 3D like in all other bodies of water, and the zone has a separate light source independent from the main world — the Creator Provincialism is right there. Sea Legs must be a really diverse yet specific spell, given that it gives you not only the ability to breathe underwater, but also apparently negates the effects of pressure and corrosion, gives you perfect sight and hearing underwater, and gives you a 60% speed increase but only as long as your feet touch the seafloor.
Eh? Sea Legs is active full-time, regardless if your feet are touching the seafloor. You always have the underwater breathing and other effects, you gain a swim speed increase full-time, but you also get the ability to "run" underwater as long as your feet are touching a solid surface (doesn't have to be the sea floor, ruins, coral, shipwrecks, etc will work too). Also, the corrosion resistance, pressure negation, etc are present without Sea Legs anywhere in World of Warcraft, too. The only thing Sea Legs gives you is permanent water breathing and the increase to swim speed and the ability to run on solid surfaces.
Sometimes you can glitch out on a flying mount and appear to be swimming in mid-air for quite some time. Thus making this trope Air Is Water Is Air and Your Head Asplode.
Fire spells still work in any place underwater.
Averted In Kingdom of Loathing. The Undersea area requires one to obtain SCUBA gear in the Final Dungeon and a Bathysphere for one's pet beastie. All tasks use up two Adventures, and weapon damage is nerfed due to water resistance.
Almost every Artix Entertainment game uses this trope. At first, it was said that Adventurers can hold their breath for a long time, but then in Dragon Fable, the answer is that a ship full of water-breathing potions exploded and dispersed the potions into the ocean. How exactly it affected every body of water is never explained.
In Kingdom Hearts: Chain of Memories, the Atlantica level controls just the same as everywhere else, despite being underwater. Ariel and the other natives still swim, but Sora, Donald, and Goofy all walk everywhere and can breathe just fine. They lampshade it at the start of the level, then shrug and carry on.
In the original Kingdom Hearts, the controls for swimming are the same as flying in the Neverland level, Fire spells work underwater, and nobody's electrocuted by Lightning spells. But then again, they ARE magic...
In Alice: Madness Returns, the HMS Gryphon sinks to the bottom of the Deluded Depths, starting the game's underwater level. It makes no difference in gameplay. You could say that it's because it's all in Alice's mind, but then, she did have to swim in American Mc Gees Alice, and that was in her head, too.
The levels set underwater in They Bleed Pixels use identical physical constants, and nobody seems to have any trouble breathing or dealing with pressure. Of course, either these are barely-lucid dreams or both their setting and inhabitants (including you) are alien to our reality. And given the game's high difficulty level it's just as well the task of getting around stays with the familiar.
SpongeBob takes a bath in a bathtub. Filled with water. (Apparently some water is wetter than other water...or maybe saltwater and freshwater don't mix?)
Patchy the Pirate sends letters down to SpongeBob and Patrick inviting them to his party. The ink of course runs. SpongeBob and Patrick go on to mention that the person who sent the letters obviously had no understanding of the limitations of living underwater, and then proceed to dispose of the letters in a fire.
Don't forget the episode where Mr. Krabs and SpongeBob think they've killed the health inspector: when they're disposing of the body, SpongeBob at first leaves the guy's head unburied because he thought that "he might need some air."
Hilariously lampshaded in one episode where SpongeBob has to go on dry land for one minute, Mr. Krabs stops him first, takes a glass but doesn't fill it with water, saying that SpongeBob should make it last. SpongeBob just drinks out of the glass, because, well, he didn't need to fill it with water, cause it's all around them.
To make a long story short, it's Zig-Zagged depending on what's funnier and more convenient. The choice of making most of the main characters bottom-dwelling species (sponge, starfish, crab) was probably done to make the lack of swimming somewhat less noticeable.
In the Futurama episode "The Deep South", Zoidberg's house burns to the ground... underwater. Zoidberg wails, "How could this have happened?" and Hermes notes, "That's a very good question." Implicitly claiming responsibility, Bender picks his still-lit cigar out of the ruins and puffs on it — eliciting a cry of, "That just raises further questions!"
Moreover, the Planet Express Ship somehow survives the crushing depth, despite the following conversation:
Fry: How many atmospheres can the ship withstand?!
Prof. Farnsworth: Well, it's a spaceship, so I would say anywhere between 0 and 1.
Bender tries to pour a bottle of booze he found in a sunken ship into his mouth, only to have it drift out and diffuse into the water.
Bender: Arrr, the laws of science be a harsh mistress.
It overall seems to be playing with the trope. The fact that the city looks like one on land is because it's Atlanta after it sank (apparently the architecture didn't change in 1000 years) and they mix in actual problems (how to breathe, withstanding the pressure) with nonsense solutions (a suppository pill which lets them adapt to the pressure, breathing devices which don't explain how they can speak clearly or why water doesn't flow into their lungs). There's also the fact that the citizens of Atlanta "evolved" into mermaids, which was supposedly sped up by the caffeine from the Coca-Cola factory. How anyone survived long enough for this to happen is not addressed.
At least they were smart enough to substitute geothermal vents for fire as their main heat source.
Justice League features this with Aquaman and the city of Atlantis. It helps that the Atlanteans are amphibious humans who live in a sealed city, and so need things like stairs, but it still doesn't answer this question: If you're amphibious, why have an entirely sealed city? Why not some places in water, and some places with air? And how do you enter the city without getting the carpet wet? And...
Atlantis had a death trap consisting of a room that filled with water... underwater in a city filled with water-breathing citizens. One would guess just shoving them out of an airlock was too much trouble.
Averted in Teen Titans. We don't get to see what Atlantis is like, but as for all the other aspects, they're either presented surprisingly realistically or just flat out not addressed. (The primary example of the former is Aqualad's hair; it just goes crazy while he's underwater)
A particularly bad example is The Perils of Penelope Pitstop, episode "Bad Fortune in a Chinese Fortune Cookie". Not only do the mobsters hold a conversation underwater while rescuing Penelope, but it's implied that the Hooded Claw, still in his boat, heard Dum Dum's joke through the water.
In the Adventures of Super Mario Bros. 3 episode "The Ugly Mermaid", Mario and friends have to protect an underwater kingdom called Mertropolis when Bowser invades it. Confusingly, Mertropolis is in an airdome but is populated by human-legged fish who need to wear fishbowls on their heads in order to survive. As if that's not enough, Bowser tries to submit the Mertropolis citizens into submission by flooding the place with water, which somehow causes them to flee in terror.
In the famous episode "Mama Luigi", Luigi GASPS underwater.
Like its source material, the short-lived cartoon Fish Police would both follow and subvert this trope, mostly depending on which would better suit the Rule of Funny. As an example, in one episode one of the villain's henchmen was pushed out of a window from several stories up; when Inspector Gil found the henchman clinging to the window ledge by his fingers (fins?) calling for help, Gil reminded the henchman he was a fish and could just swim away. The henchman let go of the window ledge — only for gravity to promptly take over and cause the henchman to plummet to the ground, Wile E. Coyote-style. (Making this an inversion of Gravitational Cognizance, in that it's a case of a character falling because they're too dim to realize they shouldn't be.)
Sharky and George played the 2-D Space aspect perfectly straight. Fish swam a few inches above the ground, or stood on their tail fins. In at least one episode, a fish fell through a trap door that opened in the floor six inches below where it was swimming.
Bubble Guppies has the same problems as SpongeBob above. They build a campfire, visit an airport, and at one point, view the moon from a telescope.
In Glider PRO, all watery environments are nothing but custom backgrounds; no special mechanics are involved. Logically, Super Drowning Skillsought to be involved, since gliders are made of paper and can ordinarily be destroyed by a single droplet of water.
Inverted for species of fish that are adapted to crawl on mud flats as well as the seabed, and can breathe air.
A rather literal version of this trope is how most air-breathing animals can breathe super-oxygenated fluid temporarily without major side effects. This is seen in The Abyss, apparently the scene where the fluid is being demonstrated on a rat was real (and PETA was quite pissed).