Pinniped
Pinnipeds sleep in the water. They may also sleep on land. In both environments, there is a danger. At least in the northern hemisphere, there is danger from polar bears on land, and in the water, there is danger from sharks. While sleeping in water, the animals go through different stages of sleep. They do some behaviors during sleep: they come to the surface occasionally to breathe, and they now and then open an eye for a period of time. The details vary in different species or groups. Similar adaptations are found in cetaceans like dolphins.