I Use 8mm closed cell underlay in the recessed (ribbed) part of the floor, this is very light, wont hold any moisture and has great thermal and sound deadening properties.
Then I loose lay 9mm plywood over that and let the furniture hold it all down, the less amount of holes you put in your van floor the better.

I have never seen the point in using Marine Plywood as the only difference is that the layers in marine ply are stuck together with a water proof adhesive and its about 4 times the price of normal ply, If you are that worried about the ply de-laminating then just varnish the back and sides of it before fitting. However in my mind if you are expecting the amount of water needed to damage normal ply then its probably not a good idea to be fitting a new floor.
