Okume
We use the Okume plywood for building iceboats. The stuff is strong, light weight and the glue waterproof.
Fir will definetely crack along the grain even if epoxied. I have tested that one recently myself. I had a jib boom made of some nice tight grained fir decking and everytime I tried to remove the jib splinters would get caught in the threads of the jibs bolt rope and would grow in size as you pulled the sail off the boom. We ended up opening up the sail slot and dropping a glass composite tube in there and then capped it with a piece of ash. Glued it in with West Epoxy and ran it thru the table saw a couple times to recreate the sail slot. The raw wood was coated with epoxy to seal it up.
A lot of people don't know it but West epoxy was originally created for the building of iceboats by the Gougeon's. They were looking to develop a way to build iceboats without the use of fastners which add weight and sometimes caused the hulls to fail in the extreeme cold conditions. It just so happens to have a lot of other great applications. Use the 207 hardner for a clear coating on nice woods that you intend to varnish.