Last week the sport of triathlon suffered another fatality during the swim portion of the Beach2Battleship race. 68-year-old Roger Ackerman was suddenly in distress. He alerted staff but soon after medical workers began administering CPR. Though Mr. Ackerman had a history of heart issues he had seemingly defeated those issues by becoming an endurance athlete who had run more than twenty, 100-mile races and was medically cleared to do the race.
When terrible events like this happen the primary suspect is a heart attack but why are fit people having heart attacks in an event they are seemingly well practiced for?
Brian Alexander has an article over at Outside Magazine exploring a phenomena called Swimmer Induce Pulmonary Edema (SIPE) which is when internal fluid enters the lungs:
There have been a number of SIPE reports over the past decade that back up MacIver’s and Bates’ suggestion. For example, during a training exercise, young Israeli special forces soldiers swam 2.4 kilometers. Of 30 soldiers who started the swim, 8 showed signs of fluid in the lungs, edema.
Unfortunately, nobody is sure just why this happens.
[Link]How frequent in the triathlon population is this problem?
From Hindawi Pulmonary Journal
"... Exercise in water places a unique set of stresses on the respiratory system. Immersion pulmonary edema with breath-hold diving, SCUBA diving, and cold water exercise has been well documented in the literature [39–41]. In a survey of 460 active SCUBA divers, 1.1% had a history consistent with the development of immersion pulmonary edema .
A survey of triathletes from the group USA Triathlon revealed that 1.4% of the members had symptoms suggestive of pulmonary edema after completing an event that contained a swimming component... 
[Link]I am going to finish off with immediate symtoms of SIPE which are listed in the Outside Magazine article:
"...MacIver listed warning signs of edema, like frothy or pink sputum and excessive breathlessness out of proportion with the normal heavy breathing of intense exercise. ..."A good athlete knows when to quit, be a good athlete. Be prepared and injury is never fun death is forever.