Try MDO. It's "Medium Density Overlay" plywood. Generally an ABX overlaid with recycled paper. Don't let the "recycled paper" part scare you, it's fairly well saturated with glue. 1/2" MDO is usually used for signs, and 3/4" MDO is usually used for concrete forms--both are outdoor exposures.Just make sure you get a plywood with an "x" designation in the name (i.e. ACX or CDX). The X stand for exposure and is designed to be used outside, the other letter designate the quality of the finish on each side so ACX is a higher quality with less defects and a smoother finish than CDX.
Very interesting idea using clear two part epoxy and Karft papper for a surface for a beaver board. That is an excellent idea, "Micarta" is what the process is called now, very strong and durable, plus economical(cheap) process to do. How I discovered this, is a friend of mine use to make knives, infact he made a skinning knife for me and used the antler's from a deer I downed for the handle, anyway he would cut square's of Kraft papper or cheepTry MDO. It's "Medium Density Overlay" plywood. Generally an ABX overlaid with recycled paper. Don't let the "recycled paper" part scare you, it's fairly well saturated with glue. 1/2" MDO is usually used for signs, and 3/4" MDO is usually used for concrete forms--both are outdoor exposures.
I agree with jspoon14, Marine ply is not really needed in this application. You'll get the same glue holding the plies together, but marine ply will generally have fewer voids in the inner plies for strength. You're not looking to keep an offshore rig together in pounding seas...you're trying to keep the groover from holing your raft floor.
Also agree that epoxy resin alone isn't enough to keep douglas fir plywood from checking. if you do glass AND epoxy then you get a lot more strength/stiffness (and cost/effort).
Edit: and in full disclosure, my beaver board is 3/4" marine ply, but I didn't buy it! My brother built it and gave it to me in exchange for using my raft on the Snake about 15 years ago. It's crazy stiff/strong and probably has 9 plies. It's now been in 3 different boats. Top surface was a little gross, so I did my own "MDO" and overlaid heavy kraft paper over the surface with epoxy. The paper looks better than the scarred up plywood. I'd do 3/4" MDO if I built one again.