Originally Posted by Mr. Pid
From the APA - Engineered Wood Association.
Marine-grade plywood is made entirely of Douglas-fir or Western Larch. The grade of all plies of veneer is B or better. B-grade veneer may have knots but no knotholes. A-grade veneer has no knots or knotholes. Both A and B grade may contain wood or synthetic patches. Panels are sanded on both faces or Medium Density Overlay (MDO) or High Density Overlay (HDO). The maximum core-gap size permitted is 1/8 inch. Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive. It is considered a "premium" panel grade for use in situations where these characteristics are required. It is available in 4x8-foot sheets of 1/4, 3/8, 1/2, 5/8 and 3/4-inch thickness. Sheets up to 5x12-feet are also available. Available grades are A-A, A-B, B-B (face-back), MDO and HDO.
Marine-grade plywood is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, it should be pressure-preservative treated to an appropriate standard.
The detailed description of veneer grades and Marine-grade plywood is contained in Voluntary Product Standard PS 1-95 Construction And Industrial Plywood.
Every piece of fir plywood I have ever handled, be it exterior grade or marine grade, had pieces of the knots missing on the surface veneer. If it is missing on the surface veneer then it is going to be missing from the interior veneers as well. Those missing pieces of knots are air pockets that will eventually draw water vapor in to condense as moisture into the wood structure in a freeze thaw environment.
The last piece of 3/4 marine fir I cut up had chunks of knots come out of the veneers since the glue did not adhear them to the other veneers. If the knot is loose then there are voids for water vapor to condense into and start rot.
I have only used a few sheets of marine grade fir and none of them met the standard above. The mills must think we will never actually cut a sheet of their plywood and look at the cut edges. As well my sheets of okume BS1088 fell short of the spec for the one sheet made in France. The stuff from Isreal was super nice material.
I keep my wooden boat dry stored and fix all dents and dings in the spring to prevent any issues with rot. I keep it painted to prevent the epoxy from degrading by UV and cracking. Its only about two hours of work a summer but its worth it.