Russell Roberts responds to the claim that imposing a cost on clean water would disproportionately affect the poor:
Unfortunately, left omitted is the fact that not imposing a cost on clean water disproportionately affects the poor. Because water is unpriced, there is insufficient incentive to provide it in the poorest parts of the world. That means there is very little of it for the poor to enjoy and as a result, poor people around the world die from lack of water.
Roberts points to this article, published in the Journal of Political Economy. He notes that the study finds the poor were disproportionately affected by water pricing, but not in the way most market skeptics suspect:
Using the variation in ownership of water provision across time and space generated by the privatization process, we find that child mortality fell 8 percent in the areas that privatized their water services and that the effect was largest (26 percent) in the poorest areas.
For more on water privatization in the developing world, see this post.