Regular ply is fine with a good sealer. You only need to fiberglass the joining areas for strength. Small 3-4" wide by whatever length pieces of fiber on those areas. You can do relief cuts in the wood and also bend with the grain for curve...or soak the wood and wrap/tie it around something round and let sit till dry.
well holy jackpot shit WOO-HOO
yeah i saw the relief cuts (slices) AFTER i did my seats. totaly a better route to go. then again i built mine inside my office. oh well, ill use that tip for the next boat! another tip, like how you see in 76bonneville's and mine, leave a little tack strip along the bottom inside so the upholstery guy has something to shoot staples into, and DONT go crazy with the glass there or else they wont be able to shoot the staples through it. also no sharp corners.
I used 1/2" marine ply, but like ka0tyk said regular 1/2" is fine.Marine ply is double the cost.I glass the crap outta mine.