Displacement

Archimedes figured out that if you float yourself in a full tub of water, the weight of the water that comes slopping out of the tub will be the same as your weight. Your weight "displaces" a volume of water of equal volume in order to make you float. The same applies to a kayak, The combined weight of the kayak, the paddler and all the gear on the boat will be the "displacement" of the boat. In order to float, the boat has to have a volume greater than volume of water that weighs the same amount.

A gallon of water weighs approximately 8 lbs. A cubic foot of fresh water weighs approximately 62.4 lbs while sea water weighs in at 64 lbs. A liter of water is one kilogram.