"1 gallon of water weighs 8.35 lbs"
Bear in mind, fresh versus salt and the spread in between.
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comes up with 8.34 pounds, more or less, at standard temperature and pressure (20C or 74F) for fresh water. That increases to about 8.57 pounds at 4*C.
Typical ocean salt water, about 8.55 pounds to start with. Not huge differences, but over 50-100 gallons they will add up.