A gallon of freshwater at a specific gravity of 1.000 weighs 8.34lbs. To know the weight of salt water you would need to know the specific gravity (or dissolved salt content). Sea water is typically in the range of 1.023 to 1.025 which is also the target salinity of marine aquariums. To know the exact weight of a given salinity of 1 gallon of salt water you would calculate it as this, 1 gallon of water 8.34lbs times the specific gravity of say 1.025 equals 8.5485lbs (8.43 x 1.025 = 8.5485) The weight of the water will increase directly proportionate to the weight of the salt being dissolved in the water.