How thick of plywood are you looking for? You could get a sheet of 3/8 Meranti and glue it up where you needed thicker. Otherwise a good (like A-B) fir plywood would do, Looking closely, I don't see any difference between marine and non-marine fir plywood (there used to be, the marine ply had no voids - but they have dumbed the specification down to meet the sort of wood they can get now.)
Michael