As a pontoon boat owner (salt water) I know a little about plywood out in the
weather. The real trick is to be very sure the edges are sealed well. That is
where plywood starts to fail. Start with a good outdoor grade paint, thinned a
little and soak the edges. I used a small roller and kept going around the
sheets until it couldn't possibly take another drop. Let that cure up well and
give it another coat. My plywood deck is still OK after 14 years.
I did use MDO.