This is where I ordered my BS1088 Marine ply from. It’s pricy, but you’re also paying for the aesthetics of it when finishing with varnish. That said the stuff doesn’t check like Douglas fir does.

https://www.worldpanel.com/marine-plywoods

I replaced my transom and bulkheads using thermolite.
https://www.merrittsupply.com/produc...mo-lite-board/

coosa is not available here. I’m told this is the same thing.

my experience:
Machines like wood (cut jigsaw, sanding, etc)- but itches see below
much lighter than wood
bonded well using polyurethane adhesive between plys.
fiberglass stuck well to it. Put a coat of resin on then apply glass and coat out.
I cut 2- 2” squares and bonded with loctite PL adhesive clamped overnight.
Then stuck in water bucket. It floated so I had to put lead ingot on it to weigh it down. Pulled it out everyday for 2 weeks to verify buoyancy and adhesion.
after 2 weeks completely submerged. Still buoyant, no obvious sign of water saturation. Could not pry apart


Pros-
lighter
stronger
more water resistant
termites won’t eat it (issue here)

cons-
costs 2-3x here what wood does- marine ply $100+ for poor quality, thermolite $250-300
itchy like fiberglass in machining
the shiny side needs sanding (if it has one) before fiberglassing.