I'm going to use marine grade, found a place local for $78/ sheet for 3/4 and $58/sheet for 1/2 (a little more than double the price of treated plywood here locally). I'm going to do the back of the seat in 3/4 and the rest in 1/2, this saves ~60lbs over what is currently there (plus the additional weight of the water logged wood that's there, not much, but it's something.

I use 1/2 treated plywood, glassed it all , resin is cheap. light and I wont have to touch it again

Don't use treated in marine applications especially if you plan to coat with resin, use a good marine ply. The best way to apply resin is to do a hot coat, let that cure then apply another coat with a waxing agent. I go a bit further with my stuff and fully encapsulate the structure with ounce and a half mat. The biggest problem is the edges where the wood can breathe and open up the surface coat.

I used King Star-Light to re-do my seat and engine cover/hatch. Stuff is flexible, takes staples, will last forever and I paid $86 per sheet locally in Phoenix.