Q. What is the difference between marine grade plywood and regular exterior grade plywood?
A. Both types of plywood use an exterior grade adhesive, meaning that the adhesive does not lose its strength when wetted or heated to a reasonable temperature. However, the difference is the interior ply quality. Marine grade can have very few defects in the interior plies, which means that there will not be any voids, so nails or other fasteners will be in solid wood.
Further, there are no voids (no holes) and limited knots so there is very good adhesion throughout the entire ply. This improved adhesion can be an advantage in construction where the high strength is important and delamination cannot be tolerated. Also, as you might imagine, when sawing marine grade into smaller pieces, the edges will be solid wood without voids, which again can be an advantage at times. The real question is whether these improved quality features are necessary and worth the money.