A 63-year-old firefighter and Tinman Triathlon athlete died Thursday from injuries he sustained during rescue watercraft training earlier this week off Diamond Head.

Firefighter Clifford Rigsbee, a 21-year veteran with the Honolulu Fire Department, was on a sled being towed and operated by a fellow member of his engine company at about 10:45 a.m. Tuesday. The exercise was being conducted at the surf break known as Suicides.

The cause of the accident is under investigation, according to Fire Chief Manuel Neves.

He said the operator of the watercraft looked back and saw Rigsbee floating unconscious in the ocean.

He was rescued and brought to Makalei Beach Park by fellow firefighters, including an off-duty firefighter. Rigsbee was treated by firefighters, Ocean Safety personnel and paramedics and then taken in critical condition to Straub Clinic & Hospital, where he died late Thursday night.

The operator of the watercraft also was treated at a hospital and released.

Both Rigsbee and the watercraft operator are assigned to Engine Company 7 stationed in Waikiki.

Rigsbee joined the Fire Department in July 1995 and was promoted to Firefighter 3 in October 2004.

“Cliff was also a well-known and nationally ranked triathlete who had won several Tinman Triathlon competitions,” Neves said.

While off duty in October 2012, Rigsbee rescued an injured hiker on the Koko Crater Trail and carried her down from the top of the trail to waiting fire personnel and paramedics, Neves said. Rigsbee received an HFD Certificate of Commendation for his actions.

Funeral services are pending.