The water's just fine, everybody in the tub. / I love make waves... and watch the faucet run. / I love the way that I feel when I'm done. / There's nothing much better for good clean fun than rub-a-dub-dub, everybody in the tub. Rub-a-dub-dub, everybody in the tub!
A work promotes the idea that taking a bath can be fun. This is usually in shows, books or other works for very young children, though there's no law that says that it has to be. It may be because one of the characters on the show Hates Baths, or is simply a reluctant bather; in other cases, it may just be that they're trying to get in their daily Aesop.
This is a relatively new idea - there was a time when bathing was actually considered unsanitary or potentially dangerous for certain religious reasons. It only really came about when it was more universally recognized as healthy and parents needed ways to encourage reluctant kids to take their daily or nightly bath.
If you were thinking something dirty about the meaning of this trope, forget it. This is about characters having fun getting clean. It doesn't have anything to do with anything dirty, other than any mud or grit that might be removed.
At the start of the film, "Tree of Might", Goku and Gohan are seen taking a bath together.
In one episode of the Buu Saga, After Kid Buu is killed by Goku, and Majin Buu is reformed, Goku is seen rough-housing with Goten and Trunks (and eventually Gotenks) as the three of them take a bath together.
Zatch from Zatch Bell! loves taking baths. One episode even has him innocently asking Tia if she wanted to join him for one. Tia, being the psycho she is, outright refuses.
In One Piece, the Straw Hat Pirates' ship, the Thousand Sunny, has a huge bath. Word of God is that the male crew members sometimes get in together for a rowdy "bath party". Chopper is often invited to bathe with the female members, but he prefers to bathe with the guys because they just have much more fun. Much earlier, Luffy and Usopp also have lots of fun in the Alabasta royal bath by running around, splashing water on each other, pretending to train and joining in on the other guys when they try to peep at the girls' bath.
In the Toy StoryPixar ShortPartysaurus Rex, Bonnie of the third film takes Rex with her into the bathroom for her bath and they have a good time with her bath toys. After that, he gets left behind and ends up helping the other toys to have a party. An officially licensed clip can be found here.
In D.W.'s Guide to Perfect Manners (later retitled D.W. Says Please and Thank You for paperback), the title character is shown in the bath holding a toy mermaid in one hand, a rubber duck in the other, and splashing Mrs. Read and Pal. She states that at night, it's not nice to complain about taking a bath or brushing your teeth and you can play while you get clean, but sometimes she plays a little too hard.
In The Lord of the Rings, the hobbits sing a bath song after arriving at Frodo's new house in Buckland, and have a lot of fun splashing around in the tubs. "Sing hey! For the bath at close of day..."
Little Critter's Just Me in the Tub depicts the things Little Critter does to get ready for his bath and then him having fun in it, though only after he washes.
In the Winnie the Pooh book, Why Does It Have to Rain? (part of the Winnie the Pooh's Thinking Spot series) shows Piglet outside in a bathtub during a rain shower, holding a scrub brush in one hand, a sponge in the other, and telling Pooh how much he loves taking a bath in the rain.
Maisy Takes a Bath, from the Maisy books, and also made into a TV story, features the main character, Maisy Mouse, trying to take a bubble bath. She keeps getting interrupted by her friend Tallulah the chicken, who wants her to come over and play. Eventually Tallulah brings over some toys and hops into Maisy's bath and the two have fun playing together. Reader reaction to this seems to vary greatly based on what cultural taboos they were raised with. In some families, though, it's not unusual two bathe two siblings of similar age together even if they're boy and girl, and in some regions a kid coming over for a sleepover may be bathed with the home kid without anyone even batting an eye. In some cases, though, the reaction isn't based on this so much as that Tallulah's behavior is considered pushy.
Huggly Takes a Bath is a picture book in which the main character, Huggly: The Monster Under the Bed, wanders into the bathroom and discovers the bathub. He pours in some bubble bath, which he believes to "magic slime" and has fun with all the bubbles and then covering himself with bubbles to make himself look like a "snow monster."
This was a kind of Author Appeal for Douglas Adams, a big fan of baths on account of being a Ridiculous Procrastinator of legendary proportions. Baths were one of his favourite methods of wasting time.
This here gives a good list of some picture books about this topic, though not all of them specifically promote the aesop.
A Playhouse Disney and Muppet show, Bear in the Big Blue House had a song called "Everybody in the Tub" in which all of the main characters sang about how great bathtime was. The writers seemed to enjoy this song / segment, because they had a tendency to whip it out whenever they needed to fill some time - it was used in at least seven different episodes of the show.
The Pajanimals, another Muppet (Creature Shop) show, has a "Bathtime Boogie" song.
One episode of Allegra's Window has Allegra teaching this to her best friend Lindi, who, being a dog, hates baths.
The Playhouse Disney (later Disney Junior) series JoJo's Circus had a song called "Bathtime" (also released on one of the show's soundtrack albums) in which the star character and her pet lion Goliath sing about how much fun bathtime is.
Another Playhouse Disney show, PB&J Otter, had Peanut and Jelly in the bath singing about how fun it was. Then, however, their little sister Baby Butter didn't want to take a bath, so they had to find a creative way to make it fun for her.
Disney Junior also has an interstitial segment in which a woman sings about how much taking a bath is and Mrs. Potts from Beauty and the Beast is shown encouraging Chip to take a bath. "Let's take a bath. A bubbly bubbly bath. Just get in, the water's fine, we'll splash and wash and have a good time... Getting clean is so much fun..."
In "The Old SwitcheRoo" on The New Adventures of Winnie the Pooh, Roo didn't want to have a bath and Tigger convinced him that baths were "the end." In the end, Kanga got them in the bath ("scrubbly bubbly") and Tigger realized the truth about baths. "You get, you get... you get clean." Roo declares that it's fun and Tigger admits that he was wrong, though he was right about one thing... it is... the end. (And the episode ends.)
Caillou had Caillou not liking baths in one episode, but then his Mom put some bubbles and bath toys in and made it fun. This was later memorialized in a song for one of the show's music albums.
An episode of The Powerpuff Girls shows that Blossom and Bubbles both enjoy taking baths. Buttercup however does not, and so she refuse to take a bath. It's only after she's run out of town by the angry citizens and a monster refuses to fight her both because she smells so bad that she reluctantly agrees to take a bath.
The "In My Bath" song from Spot the Dog has Spot playing in his bath and imagining it to be a big sea on which he can sail his boat.
When Ren and Stimpy switch places with a couple of babies of the suburban couple only seen below the knees, they see bath time is a high point (to Dad in particular) - that means everyone in the tub, including Grandpa.
An episode of Maggie and the Ferocious Beast was dedicated to bathing the titular beast after he gets muddy. He didn't mind though; he loves baths.
In an episode of Handy Manny titled "Paint Job". The tools make a mess of things and end up getting covered in paint. In the next scene, they are in a tub of soap and water as Manny cleans them up. They are heard laughing and playing during their bath.
There is a thriving industry of bathtime-related products trying to convince you or your children of just this, and in many cases doing a very good job of it. In addition to the normal bubble bath, there are also things like Sesame Streetbath fizzy colors and bath crayons.
This lists seven fun things that can be added to a child's bath.