We can run out of water in the middle of the ocean, like in the old sailor’s words « Water, water everywhere but not a drop to drink. » If you drink sea water, in the long-term, the required amount of water needed to get rid of its salt (through the kidneys) will exceed the amount of water gained by the absorption of sea water, which makes this water not drinkable. However, this is not true for cats.
All animals receive a portion of their water from food, cats included. But when the water supply from all sources is insufficient to cats, their kidneys are efficient enough to keep the body well hydrated by drinking seawater. These kidneys are able to expel the salt, allowing, meanwhile, clean water to hydrate the cat and help it survive during droughts.