I think it's quite important to use 'closed cell' foam here... regular styrofoam board generally isn't and will absorb any moisture it's subjected to - even if just through condensation.
Years ago we did a smaller boat with Ensolite - similar to the sleeping pads campers use. Indeed these pads can be an inexpensive source of closed cell foam but they come in smallish sizes. We bought our ensolite in bulk from a mountaineering co-op at the time.
Are you certain your entire deck is plywood cored? I'd not be surprised to find strategic plywood sections, but for a boat of that caliber
I'd expect to see mostly end grain balsa or something more appropriate than plywood.