I do not think that Interior plywood will survive in a marine environment no matter how well sealed. One of the items that distinguishes marine from non marine plywood is the thickness of the face veneers and the number and size of the voids. While the face veneer has only minimal impact on durability (fewer refinishings) the voids allow moisture to get to the glue lines
and allow buckling of the core plys promoting delamination.
There are really wonderful and reasonably priced 3 mm and 5 mm marine grade IPE faced and Okome faced plywoods out there and I would suggest that you try to find some of that.