Exterior grade plywood uses waterproof glue as well, but the plies may have voids which Marine ply doesn't have. 2 Pieces of exterior ply (3/4") epoxy glued bad side to bad side and coated in epoxy before paint or varnish will work. Coat the edges well with epoxy - it will soak in so keep applying it until it doesn't absorb any more.
This is probably the least expensive solution.
Living aboard in Victoria Harbour