Originally Posted by BruceB
I'd be wary filling up the hull completely with water. I know of one guy that left his cover off a fibreglass hardboat and the drain plug in and over the winter the hull filled up to the brim with rainwater.
The rollers came right through the hull due to the weight of all the water!
Trailer failure is the second problem with that method.
If possible, though, partially filling the boat and then using a jack on different sides may get the water to cover the leasking area. Nose, port, starboard.
I'd suspect something towards the back if it's filling at high speed. The nose of the boat will largely be out of the water when running at a decent speed.