I make my own beer time to time. It doesn't take 5-50 gallons of water to make 1 gallon of beer. At most you are looking at ~8 gallons max of drinking water going into 5 gallons of beer. For cooling your beer from boiling to 70 degrees F you will need cold waste water that will go through a chilling coil and not be part of the final product, but seawater can provide this function without much energy other than what it takes to run a pump used.