The water will still be there, but it will be used to benefit people and the organisms we value. There is a finite volume (*) of life the earth can support -- what we're down to is how it ought to be divided and the choice we are making is that the only organisms worth anything, are people, cows, pigs, chickens, and corn. Our population problem is an extremely unpopular topic, but by ignoring it, we will eventually destroy all the interesting biodiversity we have in the world in exchange for a monocrop of people, along with the very few organisms people tend to value, and the diseases and parasites associated with those.
(*) by weight if you will(**), not individual count.
(**) differences in body composition make "weight" not exactly accurate as some things have greater density due to the use of different minerals (hard shells or bones as a percent of body mass for example). What can be said is that there is a finite amount of stuff on the earth that can be mixed up in different ways into a finite total amount of life. The question we should ask is, what is a smart or wise percentage of that total, that we humans and the plants/animals we value, should comprise.