Two water districts in northern San Diego County are exploring the possibility of leaving the San Diego County Water Authority and buying their water instead from an agency in southern Riverside County, a move one district says could save it as much as $6 million annually. It is the first time in the Water Authority’s 75-year history that such a move has been considered by any of its 24 member agencies, officials say, and it likely would be challenged. Both the Rainbow Municipal Water District and the Fallbrook Public Utility Department have been discussing the move for several months. A lawyer for the Water Authority read a public statement at the authority’s most recent board meeting on May 23, making it public knowledge.