Epoxy! Like West System's stuff. If the mating surface in uneven use West System 6-Ten. If I was doing it, I would then cover with fiberglass mat, wet it out w/ epoxy and vacuum bag it. Very easy and now your seat won't end up in the drink with you in it ass your boat makes way to the shore or another boat, or you! This is a safety issue in a moving object subject to dynamic forces, not a wood construction project! JMO

If I was fiberglassing, I'd just grind the original floor and fiberglass the wood to the floor, run a couple of screw through it to mechanically draw them together, then continue to glass over the top.
It will never come up!
Put a good bevel on the 3/4 plywood and it will just look like a "hump" in the floor. Sand and paint or carpet over.

5200 a piece of 3/8" aluminum to the floor drill and tap the aluminum to your pedestal patern carpet over top and your done.

Plexus makes a number of very strong Methacrylate adhesives call them and see what they recommend for what you are trying to do. I used some MA-530 on my boat to bond some fiberglass parts together the bond will rip the fiberglass pieces to shreds before it lets go. Many of them will also bond to metal and may be a good choice for putting AL plate on the floor as Black Baja suggested.

Wow Im convinced Black Baja must have worked on a used car a bought a while back...lol... That genus Black.. Never would have thought of that.

Chris