To be honest, I don't think this qualifies as a phobia or irrational fear. It seems like a legit thing to be scared of, to me. The only thing, that even conceptually, gives me chills down the spine is the idea of slowly sinking into the abyss of the ocean. Just the idea that light doesn't penetrate too far down, and after a while you will be in dark, unknown depths... with unknown creatures sensing your presence around you... *shudders*
I drew this picture just now. Around that idea, but with a submarine and a Cthulhu like creature underneath it.
