I did the same project on my 1984 Starcraft a few years ago and used 1/2 plywood treated with epoxy to seal it.
Used pop rivets to hold it down
At the time I used carpet to cover it, it was marine carpet from Cabelas. Did not wear very good and took a bunch of stains on it.
This winter I used Hydro Turf to replace the carpet.
It is a 1/4 in very light weight close cell foam that is glued down.
Found it on the Duck Hunter Forum and so far it is pretty good.
Will know better when it is down for a longer time.