If you get plywood rated for a floor, it will be better quality - smaller voids. I don't know about the glue though.
You may want to consider putting a couple of coats of something like West Systems Epoxy on the plywood. You can put a few coats on the plywood - all 6 sides, then install the floor and re-treat any spots where fasteners were used. If the floor is coated well, no water will get in.
[edit] fcooper beat me to it. The West Systems stuff is pretty good but pricy. It is a product that a boat builder would use. I believe that epoxy will have better adhesion and strength vs fiberglass. I'll warn you in advance that a gallon of the West epoxy and hardener will cost way more than a few sheets of plywood though.
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