Doctors tried to revive Harry Byatt after he was pulled from the water, but he was pronounced dead on arrival at hospital.
Harry Byatt was found unconscious on the seabed at a depth of about 30m on 6 August.
The alarm was raised five minutes after he went under the water using a mask and monofin, said a spokesman for the Peligoni Club on the island of Zakynthos.
Mr Byatt was brought back to the surface and given CPR on the rescue boat.
"Three doctors, two guests from the UK and one local doctor, and an army medic from the UK, did everything they could to revive him on site whilst awaiting the ambulance," the club spokesman said.
"However, on arrival at the hospital he was pronounced dead. We are working closely with the local authorities in their investigation of the accident.
"At this time our thoughts and prayers are with his family, friends and colleagues."