"Carbondive are scuba tanks and have no issues with water."
The issue with these tanks, is that they cannot be easily inspected fully, and because of the design are, more susceptible to damage.
They have 3 layers.
1. A lightweight "thin" metal liner
2. A carbon fiber wrap
3. Protective Glass fibre layer
Because the metallic liner is thinner than regular tanks, it must have tighter tolerances with respect to corrosion damage.
The carbon wrap is not structurally bonded to the liner, so their is an interface. If at anytime moisture gets between the carbon and the liner, galvanic corrosion starts. You cannot see this visually because it's hidden
The top layer of glass is there to protect the carbon - it can be repaired within limits, but if the damages allows moisture to the carbon then that's bad. Salt crystals are sharp and can abraid the fibres as well as the corrosion aspect
To inspect in service, you could obviously visually inspect the internal surface of the liner, but the rest of the material could only be inspected by Ultrasonic inspection.
Because of the make up this isn't that simple as you have effectively an air gap between the glass fibre and the carbon, and between the carbon and the liner, which can block (attenuate) the ultrasound signal, and of course phase transitions as different materials have different properties for sound travel
The upshot of all of this, is you would need to have these inspected by a test house that has significant experience in aerospace carbon component inspections
I don't believe the costs associated with these cylinders as well as the potential risk of in service defects makes them an good and certainly not and economical alternative.
Because of the tiny quantity of these cylinders in service over a short tie (compared with traditional cylinders) you can't predict a life span based on evidence
For interest
Luxfer publish this document While it isn't specifically Scuba cylinders, it does show in detail the manufacturing as well as teh damage and repair limits
