Water to be exact weights 8.34 lbs per gallon or 1 gram per 1 cubic centimeter. What is missing in your equation is the weight of the tanks themselves and all the infrastructure to support and secure those sloshing tanks that crash down off the waves with the healing boat at sea. One argument against the water maker used to be weight. Today watermakers are lighter, take up less space than the once did but water stayed the same size and weight while the tanks keep getting bigger and bigger. As long as the correct location and the power considerations are taken care of there is no reason beyond $5,000 to $10,000 in upfront cost to not put watermakers on the boats.