Originally Posted by Psyfalcon
You do have RIBs and inflatable kayaks. The air does provide some structure for them.
I've seen those and other low pressure/high volume bladder implementations; they are not quite so good at tensile strength. This is reflected in what happens when an inflated tube is deflected: overall volume is decreased increasing internal pressure while longitudinal tension is greatly increased on the surface the the air form opposite the deflection.
In the past I observed thin pressurized air lines becoming very inflexible when approaching their rated limit. My thought is to use a coaxial kevlar braid (like in those finger trap puzzles) tube with an inner tube like a bike tire's tube. The butyl should be able to hold upwards of 300 psi as long as their is a structure to hold it. The coaxial braid ensures that internal pressure reinforces and distributes loading forces. Kevlar might even be overkill; you might be able to use high end fibers like Dyneema or Spectra.
Unfortunately, even testing this concept requires manufacturing things that haven't been made before. I do expect that for the above described application that diameters in the 0.5"~1.0" range will prove optimal.
That is if it works at all.