Despite the fact that these marine mammals thrive in the ocean whales cannot live in freshwater environments, at least not for long periods of time. In fact all known species of whale live and thrive in saltwater environments rather than freshwater, and there are a number of reasons for doing so.
Herein, how do marine mammals sleep?
Observations of bottlenose dolphins in aquariums and zoos, and of whales and dolphins in the wild, show two basic methods of sleeping: they either rest quietly in the water, vertically or horizontally, or sleep while swimming slowly next to another animal. At these times, the mother will also sleep on the move.
Do whales need to drink water?
Marine mammals have to get water from what they eat or from the sea. Whales actually drink sea water. I suppose that other marine mammals do this too. They have very big kidneys that can get rid of the salt without losing a lot of water.