Superman might be dead, but when one Green Lantern dies, he gets replaced. I'd guess the same could be true of Wonder Woman unless something happened to all of the Amazons on Paradise Island.
Now that I think of it, that might be a good story; The death of the last superheroes and mankind trying to handle things on its own after getting used to having them around.
OF course, this being the extreme future, it's also possible that they were already synthesizing water for some reason, maybe because not every area of the world has water in abundance.