8.345 pounds per gallon.
A pound is 16 ounces. If water was 8 pounds per gallon, a gallon of water would weigh 128 ounces.
A cup is 8 fluid ounces. Two cups is a pint, and a quart is two pints. A quart = 8 * 4 = 32 fluid ounces. A gallon is 4 quarts, therefore a gallon = 4 * 32 = 128 fluid ounces.
Therefore, one fluid ounce of water approximately weighs one ounce.
The actual number is 1.04 oz / fl oz, assuming that the discrepancy is due to our accuracy of measurement improving over time.
Likewise, a pint of water (2 cups, or 16 fl oz) weighs about 1 pound.
This is similar to the metric standard, where 1cc of water = 1ml = 1g.