Where to start? I agree with TrentePied's
advice 100℅ . "Marine grade"
plywood sounds great, but it is not resistant to
rot. The only difference from regular
plywood is that the plies are assembled more carefully to prevent voids. Same
wood, same glue. Oh yes, the other difference is much higher cost. For hulls, use
marine; for
interior construction, I don't think the slightly greater strength is worth the cost. As for
rot, I suggest reading Dave Carnell's "Chemotherapy for rot.". Also, I just checked the site recommended by TrentePied on dry rot. Very good information, although aimed at land- based structures rather than
marine.
Salt water does not promote rot; fresh
water does. That is why shipbuilders used to build in
salt boxes or bags of
salt where they expected fresh
water, such as rain, to leak in from the decks.