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I must state this is not "my" idea, rather learned from several veteran whaler owners who have used this technique with good success.

You can make the hole slightly larger. The key is the allen wrench and making a cavity.

Gorilla glue expands ALOT so don't fill the hole to the very top. (Squirt out a bit on some cardboard and let it set so you can see by how much it expands). You want the glue to ooze out a bit. If you tape off the area really wide, it will help keep it from getting on the deck. When the glue expands, it will bond to the foam and fiberglass deck so you will have more area for the screw to attach to.

Aside - 80% of my "advice" is from all the reading and talking to whaler owners who have done work on their boats.