According to Discovery of Sound in the Sea, animals use sound to communicate underwater. They emit different types of sounds and gather information on their surroundings by the echoes from those sounds, a process called echolocation.
Animals use sounds to communicate underwater for a variety of reasons usually involving mating, territory and group structure. Sound helps stragglers to locate their group. Some species of marine mammals use echolocation to find prey. Even fish use sounds, such as grunts and clicks, during reproduction or for self-defense. For example, oyster toadfish live in a murky environment, and they makes sounds to attract mates because of the low visibility.
Dolphins communicate underwater through a series of chirps, clicks, squeaks, whistles and barks. In echolocation they make sharp clicks, which helps them to not only find prey, but each other as well. They whistle to keep in contact with other dolphins, signal danger, identify themselves or call for help. On the other hand, killer whales wail in order to keep in contact with each other. Blue whales emit long calls that last for nearly half a minute. Fin whales have one speaker while the rest answer. Scientists found that captive dolphins and whales make more sound underwater than their free counterparts.