Many of us have been swimming with sting rays, including my children. This was not one of Steve's typical adventures. If he'd died from an attack of a croc, snake, aligator, etc. then I would have to say, "Well, he knew the risks."
Thing is, he died from a freak accident. No one in Austrailia has died from a sting ray "attack" since 1945. You have a much better chance of being hit by lightning than dying from a sting ray "attack". If the string ray had simply stung him, there would be no issue, instead the barb slipped past his rib cage and pierced his heart. That is indeed a freak accident and a tragedy.