If you include the water used for sanitation before hand, cooling the wort, and cleaning up everything afterwords (including the fermenting tanks after fermentation is done), my guess would be at least 3-4 gallons of water used for each gallon of drinkable beer. Even if you used seawater for chilling, you would have to rinse with plenty of fresh water to avoid corrosion. Any place where water is a seriously limited resource, I can't see the economics working out.