Hey, make sure it is all fir and not hemfir as hemlock rots easily. The japanese like it for
wood working because of the whiter color but you don't want it on a
boat. For cdx fir ply the 3 lamination 1/4" /6mm size isn't a bad risk as you can fill any voids in the outer 2 laminations. Voids in ply are a trouble spot in the
marine environment as they can allow moisture vapor to collect and pool starting a
rot spot. Often meranti
marine ply like Hydrotek and aquatek is cheaper than marine fir. For sure the grain is often better.