I had the same issue and replaced the core. You need to track down the leak which most likely the pedestal or pedestal guard feet. It was a pretty simple process but takes time. First remove the pedestal which requires you to go underneath and take off the steering cables, disconnect the compass wiring and instrument wiring if any. Then remove the throttle and shift cables which I took off the engine. Once the cables and wires are removed unscrew the bolts. You should be able to pull the pedestal off the boat. Next you need to cut through the fiberglass portion of the floor. You can use a circular saw or router with the blade set to about 3/16ths of an inch. If you have access to one of those Dremel type saws even better. Try and remove the floor in one piece if at all possible. It may take a bit of effort with a sharp chisel. Once you have the floor off you will see a plywood core that is cut into small sections . Remove the wet ones if not all of them . Replace with same thickness plywood. I covered the new plywood with epoxy, making sure I sealed the edges. Next dry fit the new wood in. Once that is done epoxy the new plywood in place. I then put another layer of epoxy on the wood and the underside of the fiberglass flooring and laid on top of the wet wood. I then placed heavy weights on the floor. I used car batteries. Let it set up. Then use a good epoxy filler to fill in the seams. Once done, you can reverse the pedestal removal process " don't forget to cross the steering cables". Sand well and paint with the nonskid of your choice.
