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.