When we fix water leaks at the dealership where I work we use a 3m product it is called 3m flowable seam sealer it comes in a tube you will need the tube applicator tool to apply it.The stuff is amazing it hardens quickly.It would be better to apply it to the upside surface if you can flip the boat over to allow that to be the upside that is ok(flowable it would form drips as it hardens on the bottom side).It should do the job for you and can be found at auto parts stores like sanels.polebreaker