Fiber-reinforced composites are basically analogous to spaghetti dispersed in glue. Really strong in tension, when the (in this case, carbon) fibers are taking nearly all of the load. In compression, the fibers have nearly zero resistance to flexure, and the load is mainly being taken up by the glue (the matrix, in composite-speak). When the fiber/matrix interface fails due to fiber flex-induced shear force, you're done.

I just can't imagine why a submersible structural designer would select composites for this application. IMHO, this project was doomed from the moment of that design decision, even setting aside all the other idiocy.