"Do not sandwhich in fiberglas
"Do not sandwhich in fiberglass, one small leak and the plywood even "marine" grade will rot.
Here is an article I robbed from another chatroom.
"Marine and exterior plywoods have similar glues but there are two primary differences that apply to all marine rated plywoods in comparison to exterior rated. . 1. There are at least 2 more plys in marine plywood over 1/4" thick. 2. The interior plys are fitted better resulting on fewer voids.
For domestic fir plywood that is as far as it goes other than the face veneers are usually A/B on marine and A/C on exterior. When it comes to imported BSB rated plywood like meranti and okoume, there are more differences. There will be 4 to 6 more plys glued together under more pressure, even less voids and better face veneers that do not raise grain or check as badly as fir and pine faced ACX..
The net result is that marine plywood bends more evenly to a fair curve than exterior, has a slightly better ultimate strength and there is less possibility for water intrusion into interior voids.
From a practical standpoint, if the boat will be trailered and stored dry, has no critical curves and weight is not an issue, good old ACX will do fine. OTOH, if it will soak in water for its lifetime, has has fine lines or weight is critical, marine is the better option.
I include weight in this comparison even though ACX and marine plys of the same species weigh about the same with marine fir being slightly heavier than ACX fir.. The reason being that some imported woods, particularly okoume, are much lighter for the same thickness as the fir and pine used in ACX.""