Regardless of whether you have fish people, merfolk, shark men, or some other variation on aquatic sapients this wiki hasn't come up with yet, because Atlantis Is Boring they all have one thing in common: they sometimes have to interact with non-aquatic species. Unfortunately, some of them only have gills and would suffocate if they tried to go to the surface unassisted. Others have issues with drying out or would be crushed by their own weight without water supporting their bodies (this is an issue for beached whales in real life).
The obvious solution? They bring water along with them.
The inverse of an Artificial Gill, which allows terrestrial organisms to breathe underwater. Compare Artificial Limbs.
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Anime and Manga
Teacher Iruka in Jewelpet Sunshine is a pink dolphin who gets around inside a fish tank, which the students take in stride after being freaked out the first time.
Fujimoto in Ponyo on the Cliff by the Sea is forced to take a back-mounted machine with him whenever he ventures onto dry land, as it carries a vat of salt water that he must spray around himself to keep himself humidified. Still, he is biologically human, which requires him to wear a bubble underwater.
In the Tokyo Mew Mew spinoff manga Petite Mew Mew (an Alternate Universe in kindergarten), all the girls show characteristics of their infused animal DNA — in Lettuce's case, her lower half is a porpoise tail. She is always shown partially submerged in a body of water, such as a wading pool or a giant fishbowl.
Prince Namor the Sub-Mariner can breathe on dry land, but he's a mutant. The other denizens of Atlanta have to wear water bowls when they're above the surface.
A group of Atlantis characters popped up in The Savage Dragon from time to time, with aquatic "breathing" gear to use to go on land for an invasion. This was a Running Gag in which the gear always malfunctioned in someway, killing all of them.
Minion from Megamind is a piranha-like fish in a robotic gorilla suit.
Fish Out Of Water from Chicken Little wears a scuba helmet filled with water in order to interact with Chicken Little and the others.
Gill, Deb and the other captives in the dentist's fish tank in Finding Nemo use the fact that they've been put in clear baggies of water while the tank is being cleaned to make their escape to the sea. This involves bopping the baggies forward through a building and across a busy roadway. Good news: they succeed. Bad news: they have no idea how to get out of the baggies.
Film — Live-Action
Variation in Pirates of the Caribbean: At World's End. Negotiations are taking place on a sandbar, but since Davey Jones is cursed to be unable to set foot on land, he's standing in a big wooden bucket of seawater, with several others visible behind him.
In the Hellboy films, psychic fish man Abe sometimes wears a water goggles and water tanks when he is out of his tank. The comics explain that this is more to keep him from drying out than to help him breathe; he has both lungs and gills.
Folklore and Mythology
The kappa, a river-dwelling creature of Japanese folklore, has a deep depression in its head that is full of water, which is the source of its power. If you bow to it, it will bow back, spilling the water on its head and draining it of its power.
In X: Farnham's Legend a Boron, the resident species of squid-people, is shown walking around a space station built for terrestrial species in a pressure suit filled with water.
In Un Lun Dun, there's a character called Skool who walks around in a diver's suit full of water. Skool is a sentient school of assorted aquatic creatures, mostly fish. It's that kind of book.
Downplayed in the Star Wars Expanded Universe, which notes on occasion that ships crewed by Mon Calamari are usually kept very humid. This is good for the amphibious Mon Cals, not so much for more humanlike species.
In the Sweet Pickles children's book series, Fearless Fish wears this as she rides her motorcycle around the town.
In Star Trek: The Next Generation the Benzites are a semi-aquatic race who have a special attachment to their uniforms which blows a fine mist in the direction of their faces on a regular basis so they can continue to breathe.
The Hath in the Doctor Who episode "The Doctor's Daughter" breathe a nutrient liquid, and have to wear a mask containing a flask of it while in Earthlike atmospheres.
In a variation, the Smallville incarnation of Aquaman had to drink large amounts of water frequently to avoid drying out, and heat lamps weakened him severely. He could recover from exposure to the latter by being doused with water.
The Far Side: One comic has a fish using a contraption like a diving suit with a line attaching it to a fishbowl. Another comic has two fish driving a wheeled fishbowl out of the ocean and onto a beach.
One of the aliens in Big Bang Bar is a slug-like creature in a bottle of green liquid.
BIONICLE: a few water-breathers use them in order to travel on land, such as Ehlek, Takadox, and Carapar.
Bob The Killer Goldfish from Earthworm Jim is a regular goldfish in a bowl who gets carried around by a musclebound cat minion.
The Sergetti in the Space Empires series are an aquatic species that have established an intergalactic empire with their knowledge of crystalline technology. They build spaceships that carry the oceans of their homeworld into space with them.
In Terraria, there is a literal Fish Bowl item that the player character can wear on his head as a vanity item. If it's worn as equipment (not solely for appearances), the character will start to drown. Having the character drink a Gills Potion at the same time, though, will allow him or her to breathe normally.
World of Warcraft has aquatic vanity pets. There's the variety that can be on dry land (crabs, turtles and frogs), but fish or sea ponies float around in a water bubble.
Star Control II has the Orz, who live in some form of liquid. Dialogue from other characters makes it clear that they wear airtight environment suits when visiting the Human Starbase.
Once in Two Lumps the Angry Fish goes after Snooch with a water-filled (and bubble-topped) tank.
In Futurama, the Professor's rival, Wernstrom, makes one of these for his goldfish as his entry for the science symposium.
Dr. Wasabi from Chop Socky Chooks is a tiny shark who travels around in an astronaut-style suit filled with water.
T-Ray and his mooks from Tiger Sharks are water-breathing creatures from a planet that dried up. They are now trying to conquer a watery planet; the one upon which the series take place. Since the other group of villains (and their occasional allies) cannot breathe water, they have used water filled suits at least once.
Star Trek: The Animated Series episode "The Ambergris Element". At the end of the episode two Aquans (aliens who can only breathe water) are shown on the bridge of the Enterprise wearing water-filled helmets on their heads.
Ben 10: Omniverse has the pisceans who travel on land not wearing helmets, but suits that circulate water through them.
Fishtronaut is an enviro-friendly animated mystery series in which Secret Agent Fishtronaut (named so because he's a fish who flies around in an astronaut suit) explores the world's many mysteries above and below water in a unique way.
Bull Sharkowski wears a headset over his gills in order to breathe above water.
Coach Gills from the same show, a goldfish whose bowl is wheeled around by the assistant coach.
One of the two animated Flash Gordon series had underwater humanoids use these when riding jetskis. After being implanted with an Artificial Gill against his will, Flash has to knock one out and loot his helmet.