The 2 1/2-year California drought has forced one of the San Francisco Bay Area's major water districts to virtually ban new water service in new service areas while another is considering not accepting new customers. The East Bay Municipal Utility District, serving parts of Alameda and Contra Costa counties, recently adopted a regulation limiting newly annexed developments to four service connections for the remainder of the drought. The district cannot refuse to make new connections within areas already annexed. The new regulation affects 2,091 housing units proposed by six developers. The Marin Municipal Water district, which now serves 56,000 homes in central and southern Marin County, will decide Wednesday whether to ban new water hookups. "We have reached the end of the road as far as our assignable water is concerned," said Patricia Yates, who chairs the district's board.