You will probably have to splice the keel due to lack of length but I'd start with 1/4 for that and 1/8 for any cross frames. Your best plywood for boats are 3ply 1/16 for skins, 5 ply in 1/8 and 9 or so in 1/4. I use birch ply with spruce for stringers and glue blocks when I build.