I have a hole where the previous owner put a thru-hull fitting. The hull is fiberglass about 3/8" thick. They backed up the hole with a 1/2" piece of marine plywood. They then put the thru-hull fitting through the fiberglass and the wood.

Well the wood rotted and I do not need a thru-hull fitting there (they made it for a livewell). How do I plug the hole? It is about 3/4" diameter.