[willy]

Sean something like this you have to literally make a check list and start a process of elimination.
1st determine each entrance exit you have for water on your boat. Check each one first, before doing any hull filling. Pay special attention to drain hoses, water pick up hoses, those draining your cockpit to thru hulls. Often times a cracked, dry rotted or loose fitting or hose leaks enough water to saturate or even fill the bildge during a day of fishing. Leaking live well and wash down valves hoses and connections have sunk boats sitting in the slip.
Often times you won't see a leak unless the water is under pressure as in the hull sitting in the water and the pressure against the outside of the hull, or a thru hull that is not deep enough to let water in until you are launched and have weight aboard.
2nd wait until the boat is sitting in the dark on a trailer and use a very strong light and stick it in every opening, inspection port thru hull area, suspected crack area and have someone look for the light from the outside. That one found a slow leak for me around a high speed pick up once where the bedding compound had dried and crumbled partly away.
3rd if all else fails do the water test, wait for the hull to dry in the sun or with a fan if indoors. Fill the bildge area with water after disconnecting the lead or somehow turning off the bildge pump.
You do not have to fill your whole boat, just put a lot into the under floor area with the boat tilted low on its trailer so that the water pressure is forward, if nothing tilt back so the weight pressure is to the rear of the hull.
4th If nothing comes of all this you can put plugs in each thru hull and see if the water still comes in.

9 times out of ten it is a malfunctioning thru hull valve connection or hose.
Good luck