“The best charity is water given for drinking (to the public).”

Collected by Aboo Daawood (Sunan Abu Dawud, vol. 2, p. 441 no. 1675)

A source of water, like a well or water fountain, placed in trust (waqf) for public use. Due to the scarcity of water in Arabia at that time, such an act was of considerable value. Even today, due to pollution of rivers and oceans, fresh water remains a valuable commodity in most parts of the world due to its vital role in human nutrition and its scarcity.

Source: http://blog.bilalphilips.com/2012/01/12/charity-water/