TomD
As a general rule, if plywood has black, or dark purple glue it is waterproof. Marine means it has other factors that make it waterproof even when immersed long term. Most plies will survive long term out of doors if properly finished, but the requirements vary, for instance fir ply is very difficult to stop checking.

MDO is the winner, but I have never seen it on offer, and I scour ply sources pretty extensively for boat building. I assume we use it in signs, but it hasn't even shown up at wholesalers.

Just the other day I saw an MDF mentioned as waterproof or exterior, but I assumed it was a mistake, though the guy was making picnic tables out of it. I did notice as I got deeper into the site that most of his products were baltic birch.

http://www.mobileworkshop.ca/jigsawpuzzlefurniture/mdfpicnictable.html