He was recovered unconscious from the water before being airlifted to the Royal Cornwall Hospital in Truro, where he was pronounced dead.
Two other divers from the same group were taken to Derriford Hospital in Plymouth following a reported rapid ascent, but were later discharged.
Devon & Cornwall Police said that relatives of the victim, reported to be a 67-year-old from Brentwood in Essex, had been informed. A police investigation is under way.
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