At that point, I would tie into the line, tie off as soon as possible (so I don't move the line from where he expects it) and head in the direction I believe he's going, spending as much time looking as I would if it were a lost buddy situation. Leaving a light on the line wouldn't be a terrible idea as light stands out more than the line does, but I would have to accept that he might leave the cave thinking I was trying to tell him I had left the cave, and him leaving.
Let's see, what did I learn:
- Team diving has benefits you don't even think about when you get in the water. Reenforced my desire to never solo dive, and strongly encourage those I'm friends with and new divers not to as well. Something to consider--this mill pond accident would have been a fatality had the diver been solo.
- In a cave with sharp rock, even line that looks fairly strong can slice, always ensure it's well maintained before going in, or run your own line.
- As for priority, getting on the line is always the right answer. Shutting down a valve doesn't have to be done right that second, and frankly losing the whole tank wouldn't be a huge deal in that situation. It's easier said now that I'm behind a keyboard than in the water, but that's what I would do if the same situation happened again.
- Clip your tank off again before completely leaving the restriction when diving heavy steel tanks, if possible.