Well, 1 pint of water weighs roughly one pound, so 1 gallon would be about 8 pounds.
Of course it's a lot easier in SI units, where you can just count by tens. 1 liter weighs 1 kg. Done.
A gallon of fresh water weighs between 8.33 lbs. and 8.34 lbs. Seawater, on average, weighs 1.028 times more. So, seawater weighs about 8.56 lbs.
Seawater varies in weight depending on the mineral content, so the above is just a suggested avg.