Stingray in the chest, holy crap. Usually you get stung in the leg after stepping on one. He must have been snorkeling over one and cornered it. I haven't heard if he deliberately provoked it (like he does with snakes or crocs) but either way that's a crappy way to go. I snorkel & dive from time to time, and the chances of this happening are so miniscule, what a freak occurrence.
They say it was probably his removing it that killed him. Nasty barb/serations on the stinger do a lot more damage coming out. He was quite a character who lived life to it's fullest doing what he enjoyed. The world would be a much better place if half of the people enjoyed their jobs as much as he did.