You can of course order it. It's expensive, and may not be worthwhile to you.

I do not know for sure, but my understanding is the use of special adhesives in marine plywood vs regular plywood is a thing of the past. My limited experiences seem to have born that out. I've had plywood outside exposed to the weather for years without signs of delamination.

I to built a s&g kayak with luan. Shop it around, as the different stores carry vastly different qualities. Around here, Home Depot is impossible to beat when it comes to boat building luan. It has the thickest outer plys, the most consistent inner plys, a nylon mesh between the plys for strength, and costs $7 a sheet.

And if you are very new to this, you may want to start out with luan anyhow, at least up through the stitching process. My first attempt was finally abandoned at the stitching step due to insurmountable problems and mistakes I'd made. That would have cost me a lot in okum, but since it was just luan, I lost about $40 at that point.