Thanks for the responses so far!
There are a number of details about this incident that I can't reconstruct to my satisfaction, even putting together my own recollections with the witness of folks on the boat.
The entire first stage (which was indeed a yoke) was gone from the top of my tank. No remnant left.
The geyser was (I suspect) air from the tank propelling water from the sea. Why this would be going up in the air instead of just blowing at the back of my head is beyond me.
We did find my BC inflator hose floating on the water. It was the only piece that was salvaged. It had been separated from the BC and the valve where the low pressure regulator hose attaches had been ripped out. I don't understand that at all. Also the retractor cable which holds my console to the BC was gone.
The boat was not moored at the time, so I guess it just swung around. If it had been moored I think the bow would be facing the waves instead of the stern. The prop was not moving.
Diver0001: How would you bungee the octo?
It was my own gear, alas, but my DAN insurance *may* cover it since the equipment was lost and/or damaged during a dive-related medical emergency. I'm talking to DAN about this now.
One side note: I rented gear for the remainder of the trip. Though it was fine gear, I was acutely aware of it being unfamiliar and the loss of a certain margin of safety and confidence.
Speaking of which, the first dive I took after the accident I sucked air like you wouldn't believe. So I was more shaken than I acknowledged at the time.