One of my past issues of boatbuilder mag had an article on making longer ply boards. IIRC the recommended ratio of the joint was 8:1 (in other words, 1/2 plywood would have an overlap of 8/2 (4") to maintain the strength but allow flexibility of bending. The mating edges are tapered so the joint is the original thickness. If you use marine grade plywood, the easiest way is to step it down ply by ply. I think the article was for using exterior grade ply, and they used a belt sander for the initial bevel and flattened it with a board with sandpaper glued to it. Realize this will give you a seam perpendicular to the length, but should have the original strength.
I don't remember what glue was recommended but think it was just exteriro grade carpenter glue. I will try to find the article (several years old) which was about making a 12' flat-bottomed boat. In case I can't, you can probably find more info from a DIY boatbuilding site.
It will be a challenge keeping the boards properly positioned and pressure across the splice.
Good luck:lol: