As it turns out, I am not convinced about anything, because I don't feel there is enough evidence to make a decision. You are the one who is convinced it was NOT a medical event. I will, however, reply to the point.
1. You say that people exhale enough to lose buoyancy and sink to the bottom for reasons other than medical events. What would some of those reasons be? I would think you would have to be unconscious for that to happen.
2. He was last seen during a safety stop. I would suspect a fatal shark attack during a safety stop would be noticed.
3. I would think that if he were attacked by a shark, he would have made an attempt to defend himself using his evidently unused weaponry.
4. The evidence is that he did not have any fish on his stringer. If he had, the stringer would not be next to the other gear but would have been carried off by fish attacking the contents. Shark harassment of hunters nearly always centers upon the fish that had been caught.
5. Shark attacks on divers in mid water are extremely rare. In fact, the case of a tiger shark attacking someone during a safety stop a couple years ago is the only one I know of. That makes a mid water shark attack a "zebra" in the analogy. In contrast, medical events are the most common reason for fatalities. That makes them the "horses" in the analogy.
6. As others have pointed out, the location of all the gear in close proximity suggests that the diver fell to the bottom with all gear intact.
7. Having a friend who lost her son two weeks ago to a heart attack, a son nearly the same age as this victim, a son who had no known medical conditions prior to the heart attack, I know all too well that it can happen.
So, I have an open mind, but it sure looks like a medical event to me is by far the most likely cause. If more evidence appears, I may change my mind.
So what makes you think it was NOT a medical event?