Some sleep out of the water, and some may not sleep at all. The furred or hairy aquatic mammals in the Pinniped order (like seals) have a variety of interesting adaptations that permit them to spend a comparatively long time under water sometimes at considerable depths, according to the Encyclopedia of Mammals.
However they rise frequently to breathe and emerge from the ocean to sleep, relax, molt, mate and reproduce on a sandy beach or rock above the water.
The situation is less clear for cetaceans (whales) and sirenians (sea cows and manatees). The whales evolved from land animals that returned to the sea, millions of years ago.
Though whales come to the surface to breathe air into their lungs, they can spend a long time between breaths, up to an hour in some species, and spend nearly all their lives under water. However, whales learned to hold their breath so well that they lost their involuntary breathing mechanism and must be conscious to continue to breathe. Not only would a reflex have to take care of breathing, he said, it would have to take them to the surface for air.
Also, the blow hole automatically closes and must be opened voluntarily by the whale. Whales would drown if they fell asleep or were knocked unconscious. Sea cows and manatees tend to live in warm, calm, shallow, vegetation-rich waters where they can float lethargically at or near the surface. They have an extremely low metabolic rate, do not expend much energy to regulate body temperature and require little oxygen.
Manatees may sleep or rest supported by the bottom. When they hold their breath, large blubber deposits and natural buoyancy let them float at the surface and engage in a resting behaviour, thou
gh not an unconscious sleep. 
