What material is used for the floor?

It's just plain plywood... not sure how thick.

I'd use a waterproofing deck sealer/stain. The same as I'd use on an outside deck.

As suggested you can use any kind of stain/sealer marketed for fences or decks. The other alternative is a floor/porch paint. If the plywood is pressure treated, you'll want to make sure it's good and dry before applying any kind of finish.