If you suspect water between the hull and floor you could fit small inspection hatches in your dec. One either side probably a quarter way from the stern of the boat. doing this will prove 2 things, whether there is water or/and foam under the floor.
If its just water beneath the floor you can fit drain plugs on the outside of the stern to periodically drain it out. If there is foam in there its a much larger job as some foams absorb and retain water.
Finding any leak into the hull will be a nightmare if not impossible to find without loads of time.
Another thought is if you have pedestal seats to remove these and gain access to the hull from under these then repair the holes and refit the seats.
Good luck