I will back up this post. Leaving the bottom of the wood open is the only way to go but that's only half the job. The other half is keeping the moisture out of the sub-floor and that means taking the foam out. Don't re-foam. People put pool noodles and all kinds of crap in there but that stuff just traps moisture and rots the wood.
How long can you expect a piece of plywood to last when it's got a wet sponge touching it?
I heard that PVC is the way to go for extra flotation material that lightens the boat.
BTW - I always take Ducky's advice. He makes some of the best drain plug designs in the world.