If the most powerful water officials in San Diego get their way, the county will ratchet down to a trickle one of its cheapest sources of water in the next two decades. Local officials say ongoing efforts to secure alternatives to the Los Angeles-based Metropolitan Water District of Southern California — known as the Met — have safeguarded businesses and residents against crippling cuts triggered during prolonged drought. However, the strategy of the San Diego County Water Authority to move away from Southern California’s largest wholesaler has come with a cost.