Oh a controversy... Here is what I found on eHow:
Differences in the Marine Plywood Manufacturing Process
Waterproof wood is necessary for marine use since standard plywood grades rot after exposure to moisture. The lumber used in marine plywood differs somewhat from regular plywood. In addition to Douglas fir, spruce and cedar, marine plywood suitable for boat and dock construction is made from long-leaf yellow pine, cypress, western larch, and eucalyptus. The main difference in construction is that the surfaces of the plies are better smoothed so as to reduce the "voids," the micro-spaces in between layers, to tightly adhering joints that do not let destructive moisture in. The adhesive is once again the water-resistant phenol-formaldehyde resin used in exterior-grade plywood. All parameters of finishing, grinding and cutting are done with closer tolerances than standard plywood, accounting for the very high cost of marine plywood.