ACX plywood

fireax

I would use a product called Egwood. Each layer is treated to prevent rot, in addition to water proof glue, and marine grade plys(no void to trap vapor- causing rot). Also, any wood that is used for stringers or flooring will get wet at some time (yes, even if it's fully wrapped in glass) one little pin hole or trapped vapor that condensates during temperture changes will turn into water. My belief is use a wood that will resist rot so when moisture does get by your epoxy, fiberglass or what ever else you might use you will be backed-up with good wood.

 

All exterior plywood uses the same glue as marine grades. The basic problems with using less then marine grade sheet goods is the construction and types of lumber used in the construction.

Exterior grades of ply, aren't intended to be bent in the shapes we need for boats, so the construction doesn't have to be as good, saving the manufacture considerably in materials & labor. You'll find over lapping plys, large voids, surface and interior repairs and lesser grades of wood used on the interior plys. This all amounts to ply that doesn't bend fair, once breached by a ding (and it will be) interior plys that rot out quite quickly, leaving a thin skin of wood to hide under the paint. Because of the lesser construction quality, the panel may have "soft" or "hard" areas, unseen be the eye. A chain plate or similar bolted to one of these areas can spell problems.

Anything embalmed in goo can be protected from the elements AS LONG AS THE COATING ISN'T BREACHED I've never seen any coating of any kind not get breached in some fashion during reasonable use. If you are careful to inspect the coatings, you will find cracks, dings or breaches and a quick repair can provide long service, during the life of the craft. Trailer boats have the easiest time with the necessary inspection, as the underwater areas can be looked over reasonably easily and problems can be addressed before they become major issues. Moored or berthed craft get hauled much less frequently and have the biggest chance of a breach letting in moisture for too long. And this is the key, moisture in areas we don't want, for too long, without repairs.

The advantage marine grades provide, is quality control in construction and materials used. In this country you'll find BS1088 or APA1-95 standards stamped on the product. The APA standard is still voluntary I think, but requires marine grades to have either Doug fir or western larch throughout the sheet, plus many construction requirements covering void count and size and the like. The British standard (BS1088) is built to Lloyd's standards and generally a higher requirement then the APA's.

The planking material of a boat is generally the best lumber or lumber product (plywood) used in the construction. It doesn't make a lot of sense to cut corners here, unless the craft is disposable, expected to be short lived or has other requirements necessitating lesser grade usage.