He failed to resurface August 6, while diving using a monofin - a single flipper attached to both feet - and goggles in seas about 100ft deep.
A teenage British watersports instructor died whilst snorkelling and underwater swimming in Greece, the club where he worked has said.
Found unconscious and brought to the surface, CPR was administered on the rescue boat and doctors and medical professionals took over once he was brought to the shore, a spokesman for Peligoni Club said.
"However, on arrival at the hospital he was pronounced dead".
"Harry was working in Greece as a dinghy instructor for the Peligoni Club in Zakynthos before the accident occurred".
Tributes have been paid online to the "beloved son, brother, and friend" on social media.
"Our thoughts are with his family at this incredibly sad time", the statement read.
A statement from Eastbourne College said its community was "deeply saddened" by his death.
Harry has been described as a "beloved son, brother and friend" who was "a handsome soul, always smiling, always a gentleman" who was unique and special. "The College and CCF flags will fly at half-mast at the beginning of next term".
A spokesman said: "Three doctors, two guests from the United Kingdom and one local doctor, and an army medic from the United Kingdom did everything they could to revive him on site while awaiting the ambulance".
It went on: "His parents and sister have asked us to let the college community know and to advise that further details about arrangements will follow in due course".