Hi Bushwhacker
I wish!
The difference is apparently in the glue they use to bind the layers of wood together, the quality of those layers and no voids.
When they make the sheets, there are gaps between the strips of wood they lay down to press into the sheets. These gaps will soak up water and blow the board apart. In marine ply these voids as they are called are filled.
Because of the sharpness of the curves in the front of some hulls, the ply would actually crack if you bent it to shape as is.
You can build a steamer, which is simply a box with a steam source leading into it, which you leave the ply in for up to one hour before bending to fit the hull. When the sheet cools, it actually settles to shape and stays there. A marine ply sheet must be of good enough quality to take that hour.
You can see this has been used in the backs of some plywood chairs.
Also, for at least one of the projects Im involved with, the timber has to be passed to the British Standard for Marine ply for the boat to be accepted. It needs about 10 sheets of 9mm. A £120 price hike is a hell of a lot to absorb.