Henry
Autoexreginated
Actually, the Sharn sourcebook mentions rainwater and the use of something that House Ghallandra calls "Stones of Purity" in large cisterns, which are basically magic items that cast Purify Food and Drink on anything that touches it. Remember that Sharn lies in a Monsoon-prone belt of southern coastline, so there are not one, but TWO things that are always present in Sharn - murder victims, and rain. 
