I am in the process of rebuilding my 26 oday. I have read as much as i can about plywood. So for my main bulkheads, i have decided pay the extra for marine plywood because it has waterproof glue. Marine grade not only means that it has waterproof glue, but also no voids, or empty spaces within the ply itself. the chainplates on my 26 attach to those, and since they are notorious leakers, i want to be sure that i am not replaceing them again in a year or two! For all my other cabin projects, i will use off the shelf plywood. Also, i strongly recommend doing a template with cheap 3/8 c grade plywood first. For some weird reason, i couldn't make it fit well with just using the original as a template. Maybe it was a combination of the rot, poor initial fit (the manufacture left a large gap), and maybe just plain old age (boat, not me!). Then when i get the fit i want, i will use those as a template for my expensive walnut marine grade plywood.
For an interesting lively discussioin on my redesigning my oday interior and bulkheads check the thread "cutta hole in my boat" The pix will show how my bulkhead templates look so far. They are not yet the final product.
good luck!
keith