I was in Home Depot earlier today and they didn't have marine plywood. It's been many, many years since I bought any marine plywood, and as I recall, it was quite expensive. So I'm experimenting. I bought a sheet of regular quarter inch plywood and coated both sides with Thompson's Advanced Honey Gold Water Sealer than I already had on hand.
I'll let it dry overnight and put it on the trailer tomorrow. Now I'd trying to decide . . . Do I drill a half dozen holes and put short bolts through with big washers both on top and bottom, or do I drill pairs of holes and use zip ties to hold it down and in place?
I've used pressure treated plywood with good success...the Lowes/Home Depots around here usually have it.
Another option is the oiled plywood concrete form sheets...