Well, you could block it up so the hull can't go any lower in the water, then pull the drain plug and see how far the water comes up inside.... No, you wouldn't ant to do that!
A simpler way--which the Egyptians used to level the ground upon which they built their pyramids--is to let water seek its own height. Put a clear hose on a fitting that goes through the hull, then mark how far up the water is in the hose. Nothing to it!
Jeff
"...reality is not nearly as lovely as the world of Liberal Land. No wonder so many people want to go there." - Tom Sowell