The man, who was in his 40s, suffered a medical emergency during the swimming portion of the race.
After the lifeguards spotted the man in trouble, they quickly approached him and put him into a rescue boat with the help of Environmental PD.
The lifeguards initiated CPR and rescue breathing as he was taken to shore, and before he was transported to Framingham Medical Center. He died at the hospital.
His name or exact age was not released.