The distinction is, to oversimplify it, between living in a parched desert or living next to a toxic river or a contaminated well. In the case of contaminated water, you may not even really know that your water is contaminated with, say, cholera or dysentery on a given day, you just drink it because you must.

I would also venture to guess that most people, even in overexploited nations, have access to water of some sort. So wording it as lack of clean water is probably more accurate than lack of water.