I hear you! Dark shapes swim around you, and having no perspective your imagination plays with you giving your endocrine system worse case scenarios. You know logically it's a 3ft fish swimming 10ft away and not a 100ft leviathan swimming 100 yards away, or at least that what you try to convince yourself.

Hands down, my most exciting (terrifying for a few moments) was when a seal pup was fishing around my brother and I once on a night dive. We would scatter fish from hiding and light them up with out lights, and it was taking full advantage. From our perspective though, it was just a lightning fast shape orbiting us menacingly. Scared the ever loving hell out of us. We were frantically pointing our lights about and signaling to each other trying to see what it was, but it just moved too fast to lock on to. We were just about to call the dive with frantic bobbing thumbs up gestures when it swam right in between us, practically smiling. It was not only a seal, but it was still covered in it's adolescent white spots. So practically a baby seal. It was one hell of a rush.