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.
He'll be missed, but I can't say I'm the least bit surprised. At least he went doing what he loved to do.