Ah! You argued earlier in this thread that cuben is not as prone to leaks as people think, ab, so I assumed that you were considering using the cuben as an airtight barrier.
I think your plan to use an inflatable bladder inside a cuben envelope is much more promising. Is this the basic design you had in mind (below)?
I had thought of making a heat-sealable nylon packraft/kayak in roughly that design, too. It seems that it might track better and have better speed than a conventional packraft. I think it would still be nearly as wide as a packraft at the widest point, though (in the middle, where you squeeze between the tubes). It has occurred to me that lower drag might be achievable with a large central tube with two small lateral tubes, but at the cost of much less stability:
I made a down-filled inflatable sleeping pad for my wife a while ago using the bladder-in-envelope design:
The thin, heat-sealable polyurethane film that I used for the bladder weighed about 1.2 oz/yard. I definitely would not use anything thinner. It is a tough material but it's fragile at that thickness. The envelope was M90 nylon taffeta, but I considered using cuben (and Steve Evans suggested it in the thread). However, according to my math, replacing the nylon envelope with 0.51oz cuben would only reduce the weight by about 15%, because the majority of the weight is from the bladder material. The bladder needs to be larger than the outer envelope to prevent it from being under any tensile strain when it is inflated. If you use the same bladder material I used and 1.4oz cuben, the weight of the material becomes nearly the same as heat sealable 30D nylon (2.5-3oz/yard). You can only improve upon 30D nylon from a weight standpoint if you use an ultrathin bladder material and/or a lighter weight cuben (like the 0.74 oz stuff).
Don't forget to add a (fabric?) vent somewhere (probably at least two) on the cuben, so air can escape from between the cuben and the bladder while you're inflating the bladder.