James, I considered a cuben skin boat, also, but abandoned the idea a couple of years ago when evidence began to accumulate that the material has a pinhole problem. It's actually PE (UHMWPE) fibers between sheets of PET film, and polyester film fatigues with flexion. If you cut a piece out of a polyester water bottle, fold it, and flex it repeatedly along the fold, it will turn white and eventually fail at points along the fold. Not all plastics do this. This is the reason, I think, that Richard Nisley has observed leakage through aged (creased) cuben samples.
We can see in Richard's micrographs that the cuben reinforcement layer is full of bubbles between the fibers. At these points there is no interlaminar adhesive, only a tiny air space between the two sheets of film. After some crumpling, each sheet of film gets riddled with tiny fissures, and I think leaks occur when two pinholes occur over the same interlaminar air bubble.
These pinholes are too small to be important for shelters, in my opinion. I agree with those who have said that real-world performance matters more than hydrostatic head in this case. For shelter materials, since condensation usually occurs anyway, a small amount of leakage in no way compromises the function of the shelter. But we're never going to see a successful cuben packraft, canoe, kayak, or inflatable sleeping pad, I'm afraid.