Once you start talking about weeks, drinking water is easy. A relatively clean source, a big pot and a fire are all you need.

It's also easier to move around an empty several gallon pot (or just break into a house and grab one) than it is to move around hundreds of gallons of water (a short supply for several people).

(re fire, if you build a stash, you really want there to be trees around, and pretty good annual precipitation)