Best to have layers of water supply readily available. It's easy to stock up on 1 gallon jugs from the store, like many have said you only get a year or two out of them though. You could also get one of those bladder kits that fills up your bathtub, good for 60 gallons or so. Layer two could be a few 55 gallon barrels of water (lots of places sell these, about $100 or so) storage kits, additives make them good for 5 years.
Finally, at an average consumption rate of 3-4 gallons per person/day (drinking, cooking, washing) even a larger stash of a few hundred gallons won't last long. You'll need a way to purify water from a lake or river. Like many have said, you can't beat a Berkey. A filter kit will run about $230 and can do about 10,000 gallons. If you can't swing the Berkey other places (like Cheaperthandirt.com) sell filter kits that you can use with two 5 gallon buckets, will run you less than $50 total with parts.
The bottom line is water (in my opinion) is more important than guns, food, ANYTHING. You can never prepare enough for water shortages.