I suggest that you do the plywood floor and coat it real good with spar urethane. Then maybe put a coat of Cabelas tuff coat or a layer of truck bed liner on top of that.
Plenty of poeple just let the floors lay on the ribs without securing them down. I haven't heard of anyone having trouble with them like that. BUT.. I chose to use velcro to hold mine down. I know at first it sounds funny but it really works good. I used the self adhesive 2'' wide industrial velcro from Home depot. I got 15 feet of it. Peel back the strip that covers the adhesive and put it down on the ribs. Then attach the other side of the velcro to the the one already stuck to the ribs peel back the film on that side. Once you have made sure that your plywood floor fits perfectly....drop it in. The adhesive will stick to the plywood and hold it down like you wont believe. But you will still be able to get it it out fairly easily.
I like this method for a few different reasons. First, I don't like to drill or screw any holes in my boat that I don't have to. I know that when I go to sell it it will pay off. Who want's to buy a boat full of holes in all the ribs, seats, and decks where someone built a deck or floor?
Second, Its Cheap.
Third, The velcro adds just a little padding between the plywood and floor. We all know that when plywood gets wet no matter how much you seal it it will bend and warp a little. It likes to curl up. When you hold it down with the velcro you eliminate that and when you hit a wave or walk around in the boat the wood won't bang against the hull and ribs or make any noise. If you put the foam boards between the ribs and the velcro on the ribs it will be very quiet.
The tuff coat is a rubber non-skid type paint that will work good for a wet duckhunting boat. plus it also is a good way to keep it quiet as well. It comes in different colors including tan, sand, and olive drab.