Fish

Oceanic whitetip shark.

Fish have evolved very different biological functions from other large organisms. Fish anatomy includes a two-chambered heart, operculum, secretory cells that produce mucous, swim bladder, scales, fins, lips and eyes. Fish breathe by extracting oxygen from water through their gills. Fins propel and stabilize the fish in the water.

Well known fish include: sardines, anchovy, tuna, clownfish (also known as anemonefish), and bottom fish which include halibut and ling cod. Predators include sharks and barracuda.

Reptiles

Sea turtle.
Main article: Marine reptile

Reptiles which inhabit or frequent the sea include sea turtles, marine iguanas, sea snakes, and saltwater crocodiles. Since all extant marine reptiles are oviparous and need to return to land to lay their eggs, most reptile species live on or near land, rather than in the ocean. Some extinct marine reptiles, such as ichthyosaurs, evolved to be viviparous and had no requirement to return to land.

Seabirds

Chinstrap Penguin.

Seabirds are species of Birds adapted to living in the marine environment, examples including albatross, penguins, gannets, and auks. Although they spend most of their lives in the ocean, species such as gulls can often be found thousands of miles inland.

Marine mammals

A sea otter.
Main article: Marine mammal

There are five main types of marine mammals.
Cetaceans include toothed whales (Suborder Odontoceti), such as the Sperm Whale, dolphins, and porpoises. Cetaceans also include baleen whales (Suborder Mysticeti), such as Gray Whales, Humpback Whales, Dall's porpoise, and Blue Whales.
Sirenians include manatees, the Dugong, and the extinct Steller's Sea Cow.
Seals (Family Phocidae), sea lions (Family Otariidae - which also include the fur seals), and the Walrus (Family Odobenidae) are all considered pinnipeds.
Sea Otters are members of the Family Mustelidae, which includes weasels and badgers.
Finally, Polar Bears (Family Ursidae) are sometimes considered marine mammals because of their dependence on the sea.