replacing plywood components

[MarkySparky]

Hello all, I just bought a 1988 Chris Craft 205BR Limited and need to replace some plywood components- the rear sun lounge/engine cover, part (or all?) of the floor, and upholstery trim backing. My question here is: did CC use actual marine-grade plywood or ACX in these boats? Elsewhere in the restoration forums they discuss this topic (not boat brand-specific) and it seems there are differing points of view. I can get ACX about anywhere but marine-grade is much harder to get and much more expensive. One of my navy buddies said most small boats just use ACX. Also some folks say seal the wood with oil-based paint, others say don't seal it because it has to breathe to dry out.

So, what wood do I use, how to treat it and why? Thanks!