"Chris" wrote
some of the screws essentially came right out of the floor

I'm no expert on this stuff, but here's my two cents: The screws probably
wouldn't have pulled out easily unless the wood around them was rotten.
Marine plywood doesn't have any inherent rot resistance of its own; they
typically get that from coating it with a resin of some type. When you put
screw holes in a piece of coated wood, you provide a path through the
protective layer and water can wick in and set you up for rot. (Wood rot,
btw, is caused by wood-eating fungi thingies that are always present in wood
and which tend to come to life when they get a drink of water. Pressure
treated wood works by infusing a poison into the wood that kills the little
buggers off.) If you're lucky, the rot will be confined to a small area
around the screws and you'll be able to splice in some new wood or use some
kind of filler designed for this purpose to repair it.