Done this several tim4es with elderly plywood boats: get all the floorboards out, locker tops etc so that as much of the hull is exposed. Now comes the hard work, dry the bilges right out, fisnhing off with a sponge to remove all traces of water.

Now sit and watch the lowest point in the bilges carefully. Within 5 minutes the water will be creeping back. Follow it back, using the sponge to remove it so as to be sure it has newly entred the boat.

The snag is that m/ply boats have lots of corners where water will stand and slowly seep down. so you have to be sure it is actually coming in and not just running down from somewhere.

The other point is that if the leak is on a seam you will never seal it properly from inside - you have to repair it from outside, otherwise within days you will finbd your bilges filling again!