For my sport hydros, I use 1/16" 3-ply birch for the decks and the bottom.1/8" 5-ply birch for the frames. Both Aircraft grade. Balsausa carries great stuff.
if you want something light AND strong, west system 1/32 or 1/16 ply to contest aaa balsa. but you must press it together to squeeze out as much west as you can. or you can do c/f and balsa.