When I built my little runabout fifteen (!) years ago, it was pretty easy to find suitable plywood. The boat was a Phil Bolger / Harold Payson design, and neither man was big on spending money where it wasn't required. Payson's advice was something along the lines of "well, she's glass-sheathed, so you don't need the fancy exotic plywood – just use local AC exterior ply with the boil-tested glue." He was right; after fifteen years there's no evidence at all of any kind of rot, deterioration, delamination or other trouble with Sunset Chaser's fir plywood hull.
As I'm trying to track down suppliers for the materials I need for the Starwind trimaran, I'm amazed by how much has changed since then.
Sunset Chaser's plywood came from Home Depot and was clean, flat, waterproof and nearly void-free; now, none of the big-box lumber guys (Home Depot, Rona, Lowes, etc.) sell anything like it. There's interior panelling made with water-soluble glues, there's chipboard (sorry, "OSB" as they call it now), there's pressure treated stuff that's warped into saddles, and there's the insanely rough, knot-riddled stuff they use for construction site hoarding. Most of the guys I've talked to don't even know what the standard plywood grades are. You ask for "AC exterior" and they give you a blank stare. They're labelling wood with meaningless terms like "G1S" (good 1 side) instead of proper American Plywood Association nomenclature, because the wood's so inconsistent that it couldn't possibly pass an APA audit.
There's one yard around here (Glen Supply) that carries a decent selection of ply, including some "marine" okoume. But, again, most of the stuff that looks good turns out to have non-weatherproof glue, and the okoume is a weird three-ply structure plus two ultra-thin surface veneers that, while pretty, sure ain't gonna meet BS 1088 standards.
Looking further afield, Noah's in Toronto offers a choice of.... meranti or okoume. Several grades, many thicknesses and these guys know what they're talking about, but it's just the two species – both of which present sustainability concerns, neither of which seems likely to win the Forest Stewardship Council's blessings. (And it's not entirely clear what "okoume" and "meranti" actually are; the latter term can refer to over a hundred different species.)
Searching even farther, I find that marine grade Douglas Fir is available.... if I can pick it up in Thunder Bay, or in Ohio. There's some kind of spruce available a few cities away, but nobody there knows what it's glued together with. So it looks like meranti, whatever that is, will be the ply of choice for the next little while.
I'd much rather use something local, sustainably harvested, with an FSC stamp on it. It's just that the lumber industry, or at least the part of it that has boatbuilders in mind, doesn't seem to work that way.