Brenda, just noticed you're local! We should get together sometime - I've made lot's of little "tweaks" to my Seafari over the years you might be interested in!
What type of seats do you have? Most of the deck is cored with balsa, but the section where the seats attach is cored with either 1/2" or 3/4" plywood. I installed a galley seat in mine, and the entire area under the galley seat has a plywood core.
Would suggest you drill any holes oversize, like 5/8", but don't drill thru the bottom layer of glass. Fill the 5/8" hole with thickened epoxy, then redrill with 1/4" and tap the hole for machine screws. Holds much better than the typical sheet metal screws most folks use, and the epoxy will prevent water from getting into the wood core. Denny