He was attacked by a sting ray while filming a documentary: http://www.cbc.ca/story/arts/national/2006/09/04/irwin-obit.html

Shortly after 11:00am local time on 4 September 2006, Irwin was fatally pierced in the chest by a stingray barb, while diving off Queensland's Great Barrier Reef. The events were caught on camera and the footage is now in the possession of Queensland Police.[22] He had been filming a segment for his daughter Bindi's upcoming television series. Irwin was in the area filming his own documentary, to be called Ocean's Deadliest, but weather had stalled filming. Irwin decided to take the opportunity to film some shallow water shots for his daughter's television program.[23] Some speculate that a cameraman was in front of the ray when Irwin swam over the animal which may have made the creature feel cornered and threatened. In such a case, the animal responds by automatically flexing the serrated barb on its tail (up to 10" long) upward, as it might to ward off the more common danger of being stepped on. The BBC reported that this was only the second known fatality in Australian history from a stingray attack,[24] while The Age lists it as the third.[23] The Queensland Police Service notified his family and released a statement for the media concerning his death. News of his death prompted a public outpouring expressing shock and loss. Several Australian news websites went down due to high web traffic [25] and talk-back radio experienced a high volume of callers expressing their grief [26], commemorating his passion and exuberance. Prime Minister John Howard, among many other politicians, expressed his "shock and distress" at the death, saying that "Australia has lost a wonderful and colourful son.” [27] Steve Irwin's body was flown to a morgue in Cairns, where stunned family and friends were gathering on Monday night. His wife Terri was informed of her husband's death while on a walking tour in Tasmania, and returned to the Sunshine Coast with their two children.

Stingray attack.

i am about to give you the jerkular /b/ type responce. steve erwin took a stingray to the chest. this is funny because he is the 3rd person in all of known history to die by stingray. the irony is just exentuated by the fact he was a perfessional.

after being stabbed by a stingray barb he was hauled onto a boat. during a panic, his friends pulled it out of his chest, ultimately killing him.