Apparently Human Merfolk

So you have mermaids and -men, half-human half-fish (or part seal, dolphin, whale) who come with their own set of problems. Then you have Fish People who are humanoid but scaly, think of the Creature from the Black Lagoon.

Then there's these characters. They look so human that they could wander unnoticed down a city street, but can breathe and live underwater. In fact many prefer to live underwater as that is often where they were born, and they only come to the surface to complain that Humans Are Bastards for polluting the ocean, or to do some superheroing. In fact if they are unused to life on land they may find themselves acting like a Fish Out of Water.

This can include anything from an individual who has this ability as a special quirk, to whole civilizations of these who may or may not live in an Underwater City.

Often overlaps with Walk, Don't Swim, Water Is Dry, and Water Is Air.

Anime and Manga

 * Not the mermaids themselves, but the Panthalassa race in Mermaid Melody Pichi Pichi Pitch is composed of these. Most of them, however, don't have a choice but to live underwater (until the seal on their civilization breaks, that is), and we only see a few of them, many of whom we're supposed to assume are human until we realize that they've survived instances where they should have drowned. They also have nebulous, unexplained superpowers and Creepy Cool Crosses. Huh.

Comics

 * DC Comics has Aquaman, Aqualad, and their kin
 * Marvel Comics has Namor the Sub-Mariner who has slightly pointy ears and wings on his feet as he can also fly, but looks much closer to human than fish.
 * Namor's people also normally have blue skin, but he's half-human and a mutant to boot (hence the wings which are not natural to either side of his lineage).
 * The Top Cow comic Fathom is about a member of a race of water-based humanoid Elemental Shapeshifters who have this among their abilities.

Film

 * At the end of Splash Allen becomes one of these so he can be with the more typical mermaid love Madison.
 * There's an old Soviet romantic sci-fi film called Chelovek-Amfibiya (The Human Amphibian), in which an oceanologist's son with sick lungs was given additional implants derived from the gils of a young shark. He can survive on land, but mostly sticks to the sea, where he's more comfortable with breathing. The main plot point of the movie is.
 * The Mariner in Waterworld, played by Kevin Costner, has gills behind the ears.

Live Action Television

 * The Man From Atlantis is another example. The only indication of his nature was webbed hands.
 * The titular heroine of Ocean Girl looked fully human.
 * In later seasons of SeaQuest DSV, the character Tony Piccolo is a human who was modified with gills along his sides (and probably Required Secondary Powers to allow him to survive deep-sea pressure) but otherwise looks completely normal.

Literature

 * The titular character of Percy Jackson and The Olympians what with him being Poseidon's illegitimate son and all.
 * The Secret of Platform 13 has Melisande, a merrow who is proud of having feet and continually points them out, for some reason touchy about the idea that people will mistake her for a mermaid.
 * The rifters from Peter Watts' Rifters Trilogy are modified humans (cybernetics and genetic engineering are used) who can survive in deep sea conditions. In fact, they prefer the sea to staying in their confined Underwater Base.

Mythology

 * The oceanids, nereids, and naiads of Greek Mythology look entirely human, though perhaps more beautiful than mortals.
 * This is the typical form of mermaids in the Arabian Nights and other Middle Eastern folklore; the only difference between them and land-dwelling humans is that they (and in one story, their children with humans) can breathe underwater.
 * Selkies of Faroese and Scotirish folklore wear a sealskin as a wetsuit and take human form when they shed it. The Faroese story "The Seal Wife" is about a man who steals a selkie woman's wetsuit and locks it up so that she can't go back to the sea.

Tabletop Games

 * Traveller: The Nexines who actually are human merfolk. They are a race genetically engineered for underwater mining.

Video Games

 * Everybody from Spira in Final Fantasy X and X-2. Although it's a case of All There in the Manual that everyone on Spira can, even though they didn't show in game that everybody could do it. Of the Player Characters, only Tidus, Wakka and Rikku are shown to have this ability, and they are the only ones playable in underwater areas.
 * In the sequel, Yuna, Paine and the rest of the Gullwings are shown to be able to if the player does a Blitzball sidequest.
 * In Tales of Legendia there's the Ferines, an aquatic race that looks perfectly human despite the fact that.
 * Aquaria has Li become this shortly after Naija finds him. It's a handy side benefit of your Fish Person girlfriend having magic powers, though it might only work if they're in close proximity -- Li puts his old diving helmet back on if Naija decides to leave him behind somewhere. Their son might also count.

Western Animation

 * SpongeBob SquarePants gives us Mermaid Man and Barnacle Boy, parody Expies of Aquaman and Aqualad.
 * Marina and her people (all of whom are mute) from Stingray

Webcomics

 * "Selkie" is this, too.
 * The seatrolls of Homestuck look exactly the same as any other troll except for having fins attached to their cheeks. (Fans sometimes depict the fins as coming out of their ears instead.)