I have had this happen to me twice for different reasons. The first (and most difficult to figure out) was that I had a small leak in the top of the polytheline water tank. If I ever overfilled the tank, a bit of water would squirt up and into a large recess on the top of the tank. When the boat went on plane, the water would find its way out of the recess to the back of the tank, where it would then drain into the bilge through a path that put it one stringer back from the water tank -- making it look like the source couldn't be the tank! We figured it out by being in the bilge while overfilling the water tank. The second was a bad connection in the hot water heater hoses. The heater gets very hot when underway, and it slightly melted the PEX fitting that they had screwed directly into the tank. One clue there is that the water pressure pump would run sometimes by itself. George Rankin Oyster Catcher