I never understood what all the people who were going on and on about "water intrusion" were saying, because it was never explained. But I think I understand now. Boring old A36 carbon steel is good for like 30ksi (30000psi).

The water pressure is only say 6ksi which is significantly less. That means that the steel is much stronger on its own, in every direction, than the water pressure pushing on it.

The epoxy in most epoxy composites isn't rated for 30ksi, it's generally less than 10ksi and perhaps even less than 5ksi. Epoxy on its own is capable of standing up to less pressure than the composite will see at 13000ft deep.

So the concern is that it's not if but when water manages to wiggle it's way past some of the epoxy between the carbon strands. Sure the bulk composite material can stand up to 6ksi, but if the glue that holds it together is weaker, well, the water can push it out of the way.

However deep the water gets it has reduced the effective thickness and this strength of your composite at that point, if it goes deep enough it shoots through and that's the start of a very quickly evolving collapse.