There was a similar incident that happened a few years ago. The victim actually fought off the person who managed to reach her. A medical event seems most likely but I wonder at the body position the diver took. Almost as if it was planned.
You bring up an interesting point I hadn't considered with regard to body position, Jim.. If someone actually passed out or was fully incapacitated, what is the likely position a body would be on descent... especially a rapid one? Some form of tank-down / low, I would think, probably with fins higher than the head... Body positioned vertical, with head up and fins low would require control..
Of course, the DM's perception on what vertical actually was could be off when you factor in the stress and depth he was dealing with.