I have a jon boat that actually had small holes on the back of the ribs. That was after many years of dragging the boat around.

The best stuff I found to fix these pin holes and rivets.... believe it or not is simple BONDO. Yes, auto body filler. Mix it up just like you would any other time. I have had it on this boat for over 15 years now. Rock hard, doesn't crack. It has been good stuff.