When I was more lucid, I went online with my LifeDrive and looked up some info on what happened. Apparently, he was killed swimming on top of a stingray. The Stingray got spooked and stung him with his barb, which ordinarily hurts but does not kill. Unfortunately for Steve, it had punctured his heart.
From Discovery Channel (Steve's bosses):
Steve was killed during a filming expedition on the Great Barrier Reef. While we are still collecting specific details, it was a rare accident in which Steve swam over a stingray and was stung by its barb in his chest. A doctor on board Croc One, Steve's research vessel, was unable to resuscitate Steve and by the time he was reached by the rescue helicopter he had passed away.
Now he's dealt with man-eating crocs, dangerous snakes, all kinds of deadly critters of all shapes and sizes. To die this way? It's so rare it's bizarre.