When all the water "dried up," it saturated the atmosphere to such an extent that everyone could get all the water they needed to survive simply by moving around. Eventually someone developed a way to synthesize it. After all, the recipe for water is the one chemical composition that EVERYONE knows. Without the first part of that, there's enough geniuses running around the DC universe that one of them would have figured out the second in a couple of weeks, since it would have been a major priority.