There's a finite amount of water on the planet, but there's so much that it's infinite for all practical purposes. The problem is that almost all of it is in the oceans, and too salty for drinking or irrigation unless the salt is removed, which takes a lot of energy, or in vapor in the atmosphere, and unuseable until it comes down. And too much of the fresh water is either polluted or in the wrong place (actually, the water is in the right place, the people just chose to settle in the wrong place, but you know what I mean).

Cheer up. If the global warming thing goes as the prophets of doom predict, when the Antarctic icecap melts down, all our coastal cities will have plenty of water. Lots and lots.

Peter.