I'm not entirely sure what it is...I think it's fiberglass
encapsulated. There's some solid material in the center, but I'm not
sure what it is. It doesn't seem like metal. If it helps, the boat is a
1972 O'Day 22".

When the hole was exposed, I could reach in and move the center
material back and forth a couple inches. I also let it dry out for a
couple weeks. Thanks for your reply.