The blue whale, seals and dolphins are some of the common water mammals. As they all are mammals, they have hairs on their bodies and are warm blooded. They give birth to the young ones and the mother produces milk for the baby. These water mammals are very good swimmers.
Some of these water mammals are quite amazing animals. Like, the blue whale is the largest animal in the world. While seals use their 'flippers' to both swim and walk.