Re: Geting ready to replace the wood floor in the rear of the boat. just needed help.
Short answer: Use exterior grade plywood, fiberglass cloth and mat, and polyester resin.
Longer answer: Do a search here for "deck replacement" or "soft spot in deck" and you'll find people who have done exactly that, probably on that same boat.
You'll also find out that boats tend to rot from the bottom up, so a soft deck usually means a soft transom, soft stringers, and wet foam. Not for certain, but you should check. Often people getting into a small fix find much more that's wrong and should be repaired.
Erik