Animals that live in water, usually called aquatic animals, include fish, cetaceans, certain types of turtles and other reptiles, and amphibians. There are also a number of semiaquatic animals, such as alligators, hippopotamuses and penguins, that depend on and live in water most of the time.
Animals depend on water for a variety of reasons. For fish and many other aquatic animals, water is necessary for breathing. Cetaceans, like whales and dolphins, breathe air but need water for support and locomotion. When taken out of the water, these creatures are slowly crushed by their own weight and eventually die.
Semiaquatic animals that primarily live in water depend on it for cooling down, locomotion, food, protection or some combination of these. For example, hippopotamuses are excellent swimmers, though they lumber on land and eat aquatic plants and land vegetation. Hippos are always in the water when the day is hottest, and when land predators come around, they can dive or swim away to escape.
Amphibians and a few fish, like the peculiar walking catfish, are technically aquatic but can survive on land or in water. Amphibians have a completely aquatic youth, hatching from eggs in water and growing up as fish-like tadpoles, but they later develop limbs and lungs and live a more land-based life. Walking catfish and similar species have developed lungs or lung-like structures to cope with low-oxygen water and can come out on land to migrate to other ponds when necessary.