Rationale
In the industrialized world, clean water is as readily available as air. Unfortunately, clean water is a luxury in much of the third world, and this fact is a major contributor to chronic health problems.
These regions are most seriously impacted by lack of clean water: (Ed note: I don't actually know what these areas are.)
These kinds of skills are valuable in addressing the problem:
These organizations are working on the problem and can use your skills:
These sites provide technological information: