You should probably be a bit redundant when it comes to water. This is one thing you don't want to be without.

As many people have said, the rule of 3's. 3 minutes without air, 3 days without water, 3 weeks without food. Those will kill.

With that said, carry a supply of purification tabs. The Katadyn Micropur Purification Tablets are some of the best. I've also heard great reviews on the Steripen mentioned above, esp. with the solar charger.

With this setup, you can normally boil water when you can. You can also treat the water in one of two ways if you need to be moving quickly or don't have the time to boil. The tablets are a good backup in case technology breaks down, which it always seems to do at the worse possible time.