[rich_sasha]

It is insane, but not entirely pointless. A metal submersible is heavy and negatively buoyant, whereas you want it to be positively buoyant (then add removable ballast).

The way you add buoyancy is expensive fancy aerated concrete, or something like that, and a lot of it, since it's not that buoyant. But it is pressure resistant.

By making it out of CF, you save on that cost.

Not a worthwhile cost cutting exercise but at least there is some reason for it.