I was complaining a couple of years ago that there are ways to make solar water cookers out of junk on hand, and how sad it was that nobody thinks of doing these things, preferring instead to ship bottled water thousands and thousands of miles during disasters. The shipping water thing is expensive, slow, and limited in scope by its bulky nature. It seems that the universe heard my grinching, for we now have from Unicef the SODIS Project. It's not aimed at disaster response; it's more for chronic water problems in marginal areas. It could be used in disaster scenarios as well.
Clean water is the number one health concern in a lot of the world. Knowing how to clean your own water is one of those skills that might be a lifesaver. Like CPR, chances are pretty good for most of us in the developed world that we'll never need to do this. Even so, it's a great "just in case" skill.