Marine Grade Plywood?
I guess that the answer depends on how long you want the boat to last and whether you ever intend to hit something.
Unlike 10 years ago when exterior plywood used the same gluing schedule as marine plywood, today, only some exterior grade plywoods use the same adhesive as marine grade plywood. Most exterior grade plywoods have gone to less expensive glue formulations and less stringent application schedules.
More significantly marine grade plywood has fewer voids and patches than exterior grade materials. Marine grade plywood often uses better grade (more rot resistent) flitches and marine grade hardwood plywoods are often flat cut rather than rotary cut so are less likely to form checks and ''fish eyes''.
If you were building up multiple layers of cloth and epoxy on both the interior and exterior of the plywood, were sure that no fasteners were going to pierce your moisture proof membranes and you were never going to hit anything that might crush a void, then exterior grade plywood would be adequate. But in the real world, if you are building a boat that you would like to keep for a period of time, then you probably should go with marine grade materials.
I still think that given your long term goals and your ideas about using a double hull, you would be way far ahead of the game to plank the boat with a rigid foam core and the glass over that core. In the big picture you will end up with a stronger, cheaper, more durable and lighter structure.