How do sea mammals get the water necessary to sustain their bodies?

Mammals on land have to drink fresh water to survive. How do those mammals (whales, seals, dolphins, etc) obtain the water they need? Do they draw it from the salt water they live in? If so, how do they dispose of the excess salt? Do they get it from the foods they eat like some desert dwelling rodents? How did they go from a land dwelling animal that needed water to a water dwelling animal that did not? Were the seas fresh water when they took to the water and get saltier over time?
Just wondering.