If you are from the US or Europe I doubt you will find the marine ply that you are looking for here.
Here in the Philippines there is no enforcement as to what plywood gets labeled marine. Particularly plywood from China. Even the crap is labeled marine.
Proper marine plywood is very expensive and I doubt there is a sizeable market for it here. Here people tend to buy the lowest priced products without concern for value.
On the other hand I have seen boats made from exterior glued ply and only an epoxy coating (no fiber reinforcement) last for 10 years. Particularly when removed from the water for storage.