"Food" is a vastly broader category of possible substitutable commodities than water is. And it's a stretch to claim it's entirely the work of free markets that ensures our food supply - agriculture is pretty heavily subsidised in most modern economies. I'm quite willing to accept that we would see some more innovative ways of supplying water to cities if there was less public control over it. I also agree that water should be more expensive in places that it's hard to supply to (which should help discourage too many people from living there) - but no government is going to let its voters die of thirst, and is just as strongly motivated to maintain supply as competing private enterprises might be.