FEW THINGS are more essential to 1.8 million consumers in Prince George's and Montgomery counties than clean water and sewer service. Yet, for more than 14 months, officials from both counties have stalemated on picking a new general manager to run the agency that provides those utilities, the Washington Suburban Sanitary Commission (WSSC). The impasse has, by many accounts, demoralized employees and paralyzed the six-member board that oversees the agency. Blame goes mostly to the Prince George's commissioners, who have stonewalled qualified candidates in a bid to sneak through their pet pick.