I'm admiring your work. . . . Your woodworking and fiber-glassing skills are top-notch.
I agree with the Coosa board comments, but understand the cost trade-offs. Plywood is also a lot stronger than Coosa, so you would have been going with an extra layer of biaxial fiberglass on everything, if you did Coosa. Weight would probably be about the same.
Not having read through the thread entirely, but I assume you would be filling the hull chambers with 2-part foam?

. My experienced-based recommendation is to have the liquids fairly warm (85˚F ish) and have small (1/2") vent holes at the corners of each chamber, in addition to the larger center hole used to pour the liquid in. The foam often expands up into the 'pouring hole' and traps air at the corners of the chamber, which tends to limit the amount of 'free expansion' inside the chamber. This results in a partial fill of the chamber and wasted foam. With small vent holes at the corners, the air has a place to escape.
Foam adds strength and buoyancy to the hull, but usually gets blamed for retaining water 30 years afterwards
