Marine plywood comes in many species. Douglas fir is the most common in the boat building industry. There is, however, a huge difference between "Douglas fir" ply from Lowes, Home Depot or whatever and "Marine" plywood.
Marine ply is a much higher quality or grade for a number of reasons. Look at the picture John123 posted showing the edge of the plywood. The plys are of consistent thickness, the glue - a waterproof aliphatic resin of some kind also is consistent in thickness. There a few if any voids in the plys, and the plys are all heartwood (the center of the tree trunk). No where for water to begin the decay process, in fact the only void I see in that foto is in his glue line. Go to the lumberyard and check out exterior grade (waterproof)doug fir plys, you'll find voids in the wood, air pockets in the glue, inconsistent ply thicknesses and glue line and low grade wood in the plys including knots, and sapwood (both weak points). The marine grade plys for decks and especially for transoms are worth every penny, and as you've probably already seen, they cost at least twice as much.
Regarding the Greenwood XL panels, it is heavily used by the boat building industry, but if you're finding no input it is likely because lumber yards just don't move enough of it to carry in stock and people move on to the more available products. Call a few boat builders and ask what they know about it. Boat Builder Central and Jamestown Distributing may have a lead on it. Good luck with the project.
Don't know where you are located so no clue where to shop, but good lumber yards or wood stores will have it or can get it. Unlikely at home improvement centers. These guys have good selection and their prices are about par. It's the shipping that gets you.
http://marine-plywood.us/