Marine Ply vs Exterior Ply
Marine Plywood and Exterior Plywood have a few minor differences. Both products use the same adhesiveto bind the layers. The biggest differences have to do with the plies. Marine plywood is not supposed to have any voids in the plies where as exterior grade plywood often will have voids within the plies up to 1.5" in diameter. Also, exterior plywood usually has fewer plies than marine grade. This is especially true in CDX grade of plywood. For 3/4" nominal material CDX has 5 plies, ACX has 7 plies and marine plywood has 7-9 plies depending on the manufacturer. I would not recommend using CCA treated plywood for any boating projects. It is almost always CD grade and is VERY heavy because it is sealed with CCA compound. It also is not save to have around food and splinters from treated plywood are toxic. Stick with higher grades of exterior grade or marine plywood for the best results.
FWIW, I work for a lumber yard and I am often asked to get marine plywood for clients. In Colorado it is only available in 3/4" paper coated (the exterior finish is actually a paper product). IT goes for about $100 a sheet compared to about $35 a sheet for ACX.