What's interesting is that 13 out of the 14 expired from heart failure while swimming. (The last was a bicycle crash.) Harris isn't certain why the swim leg of the race is the most deadly. While it's common knowledge that a top-speed swim in a chilly ocean or lake is unduly demanding on the cardiovascular system, Harris has another theory. Ailing competitors, he suspects, have more difficulty slowing down or flagging for help during the crowded, frenzied swim.