The discussions I've seen regarding using Luan (door skin, or flooring underlayment) for small boat building is that a lot of people do it, and it works very well as long as it stays encapsulated. It doesn't last as long if it gets wet. It's well finished on both sides and relatively free of voids. Best use would be for a boat you're not going to put a lot of investment into. It may be a good choice for a boat like this, especially if the hulls are easy enough to build - if one goes bad because the encapsulation was compromised, simply build another.

Exterior grade ply would also work fine for most applications - the thing is to make sure the grade you are using has minimal voids in the middle layers. I believe A/C has less voids than A/A if I remember right. It certainly is used in a lot of applications for plank on frame. I might be leary of it for tortured ply designs, as any voids could cause problems with bending the sheets. The glue used in exterior is better than that used in the luan.