I did a recent remodeling project with luan underlayment, and was very surprised at how much that ply has changed in recent years. What I got was 4 ply but the plys were arranged strangely, two paper thin face veneers over two core veneers with all the grain oriented in one direction. I cut all the striaght cuts with a razor knife, one score and a clean snap with the grain, two scores and a slightly less clean snap cross grain. Pretty light weight stuff barely suitable for underlayment. I have some larger scraps of older luan that is 3 ply with a cross grain core and fairly thick face veneers that might have worked for boat construction, it is much more rigid than the newer stuff. Be very careful with cheap plywood, and inspect every sheet so there are no surprises when/if you decide to buy it.

If you cannot find marine plywood locally you might have better luck with cabinet grade sheets, but be aware that the face veneers are very thin and will be difficult to scarf, and you will have to fully encapsulate the sheets to get the performance needed for your project. You will also likely pay a weight penalty for this choice, even luan is twice as heavy as okoume, but just think of all the possible species available. I know I wouldn't be too upset about a 50 pound black walnut boat for the same money as a 35 pound okoume boat.

Tom