It'd have to be a very strong shell to actually give you buoyancy above simply not having anything. Carbon fibre is strong in tension, while a material which ca

It'd have to be a very strong shell to actually give you buoyancy above simply not having anything. Carbon fibre is strong in tension, while a material which can support a vacuum must be strong in compression. The weight of any shell is going to be much greater than the buoyancy you'd gain.