Commander Sompan Meethong, 56, was the contestant No 1383 when he suffered an acute heart attack during a 1,500-metre-long swim.
He drowned before anyone could rescue him.
The swim was the first section of the triathlon, which also included a 40-kilometre bike ride and a 10-km run.
Held at the Royal Thai Fleet’s compound in Chon Buri’s Sattahip district, the contest aimed to encourage Navy staff and others to exercise regularly and stay physically fit. It also identified talented athletes for the Navy and the nation.
The Navy’s chief Admiral Luechai Ruddit expressed condolences over the death of Sompan, but insisted the competition should go ahead.
Working as the chief of a training unit under the Navy, Sompan passed a health check last month.