In truth, there is no rating for "marine plywood", it can be many things, although there are specs for certain plywood's used in the marine industry that a particular product would need to meet for certain applications. In general it means more plies and a water proof glue, the same glues are frequently used in exterior grade plywood, but there are different ratings for these too depending on the exposure level.
There are some very poor products marketed as "Marine Plywood" and many good ones, some are very rot resistant, others aren't (not being as rot resistant doesn't mean it's not a good product, it may have other very desirable characteristics), the typical product in the US is fir, with fewer voids and more layers than what the same thickness of exterior rated box store plywood would have.
The one bolt has really been a night mare for me. Heated up the area and it still won't let go. Keep on having to go back and forth for tools cause the gimbal housing and transom cut-out are not at my house and my partners have pretty much wood working tools. Tomorrow I'll bring some sharp drill bits and drill down beside the frozen bolt to relieve surface tension, spray with some Blaster and hopefully with a little pounding the Y tube will let the gimbal housing fall down.
