I recall watching a documentary about how Mexico city uses so much water that their water table is disappearing and the city is sinking. It's also a huge population (~9 million) that's going to run of water.
Hopefully we make some serious inroads in conservation and purification techniques.
Good point, Mexico City and other heavily populated cities in the world could become crisis points if the water situation worsens. It could lead to all sorts of problems as the knock on effect occurs.