U.S. Department of the Interior: WaterSMART: The U.S. Department of the Interior has a large repository of data (typically called a clearinghouse) that has information on water resources planing and management.
U.S. Department of the State: Water Issues: The U.S. Department of the State contains vast amounts of information on water as a foreign policy priority. Includes water policies and water resources.
U.S. EPA: Watershed Information: The Environmental Protection Agency maintains an enormous amount of information about watersheds and water quality. This site is easy to use and intuitive.
U.S. EPA: STORET / WQX: The Environmental Protection Agency hosts two databases, STORET and WQX (Water Quality Exchange), which house data on water quality. WQX is used to upload data on water quality and STORET is used as a "home" for that data.