Well, we are running out of freshwater, kinda.
Alot of it comes from rain and a lot of it comes from ground water (underground rivers and whatnot).
It evaporates out of the ocean (leaving behind all the salt and whatnot) and moves to land where it rains down and builds up into rivers.
In stupid(er) terms.
well we does be losing allotta us's waters without saltsies
big waters come from sky-pee and more big waters come from down under rivers then flies to the lands and falls and make water flows!