Weight could be handled with a metallic sleeve. Carbon and steel are so dissimilar that they create a charge in salt water that results in corrosion. Glass is usually used as a barrier between them. Cost is the real barrier to a composite vessel for Scuba. You would have to use a low cost process to get it in the range of those with deep pockets. I'd use a metallic sleeve with a dry glass fabric sock over it and then successive layers of dry carbon fabric. Drop it into a metal match integrally heated tool and inject resin into it. The tool would be expensive but it amortizes over part count. The sleeve doesn't have to be steel. You could use thin layer of lead if you wanted. You want it for density and forming cost. If you don't use steel/alum or other common aero materials, you'll have to make sure your resin bonds to it, but that's easy to test for. In the end, you have a comparable weight, expensive, high-pressure cylinder.
