The leading water-related legal treatise is Waters and Water Rights (LexisNexis), available in print and on Lexis (ENVIRN;WTRRTS).
The Water Librarian's Home Page provides an extensive collection of links to water-related web sites. The National Agricultural Library's Water Quality Information Center answers questions and provides lots of information about water pollution.
The U.S. Geological Survey provides Water Data for the Nation concerning the "occurrence, quantity, quality, distribution and movement of surface and underground waters" in the U.S.
The Safe Drinking Water Act and related materials are posted at www.epa.gov/OGWDW/sdwa/sdwa.html.
The EPA's Surf Your Watershed lets you look up the relative cleanliness of water in a particular region. The EPA's Safe Drinking Water Information System (SDWIS) lets you look up the contaminants in the public drinking water for a particular location in the U.S.
Foreign Laws: FAOLEX summarizes (and often provides English translations of) the water-related laws and treaties of FAO member nations. See also "Foreign Laws."