Personally, I wouldn't spend the extra $ on marine plywood, regular old pressure treated plywood will do just fine. Marine grade plywood actually isn't any more rot resistant than plain old exterior grade plywood, it's only difference is in strength. It's made with all veneer (surface) grade layers throughout, so it's got no internal knot holes to weaken the structure. It's made from the same type of wood, and uses the same type of glue as regular plywood.
I did a ton of research on plywood when looking for a material to re-build the transom in my boat this summer, and those were my findings on "marine grade" plywood.