I have a standard deck with stained treated board covering. It is on the second story and i now plan to put another deck under this one. It is not essential that no watere get tothe new lower deck but i want to seal the upper deck essentially.

As the exisitng floor is ok I plan to use plywood over it for the new floor. Should i use marine plywood or simply exterior plywood. Or do you have another idea for this project?