Re: Who sells marine plywood?
For inside the boat not exposed to water, a both-sides good hardwood plywood saturated with a penetrating epoxy would be more than good enough. 3/4" plywood is better than 1/2".
This may also be good for the transom work but double 3/4" and add a few layers of saturated fiberglass fabric.
1970 Havsfidra 20 by Fisksatra
On the Delaware River at Fox Grove Marina Essington PA