Matt Hill, 37, was walking through the Fort George Military Cemetery in April with a friend when he spotted a cross for a marine.
Mr Hill became connected with the marines when he was 13 as a cadet and has been a leader with the Guernsey Royal Marines Cadets for nearly 20 years.
The cross piqued his interest.
‘I feel the marine corps is in my blood,’ Mr Hill said.
The cross detailed how Royal Marine Thomas Rees, of HMS Nelson, had drowned in St Peter Port Harbour on 19 June 1931. It was in a poor state and it broke when Mr Hill and his father, Alan, tried to clean it.
Having checked with the St Peter Port constables and the Commonwealth War Grave Commission, which looks after the cemetery, they took the cross and restored it.
Despite its age, the teak is now in good condition, after being oiled repeatedly.
The original dark lettering faded when the wood was oiled. That has now been painted white to make it clearer.
The grave surround was also in a poor state of repair, but the Pandora Hotel has donated some wood to restore that.
The cross also spurred Mr Hill to do some research on the man whose grave it marked.
Thomas Edwin Rees was born in Pembrokeshire in 1907.
He was one of six children of Henry and Theodora. He was the second youngest, with older siblings Henry, Eleanor, Clement and Minnie, and younger brother Cyril.
Thomas enlisted in the marines in 1925 and served on HMS Ramillies, HMS Renown and HMS Nelson. In June 1931, the battleship HMS Nelson was visiting Guernsey.
Thomas spent an evening in the Brighton Hotel with his shipmates, enjoying three bottles of beer.
They moved on to the Channel Islands Hotel for another drink, but were still relatively sober when going back to the White Rock at about midnight. But as he tried to return to HMS Nelson, he ended up in the water.
Witnesses tried unsuccessfully to rescue him, then lost sight of him. They thought he had managed to get ashore, but he was later found drowned in the harbour.
He was described as having been an active man with a happy disposition.
He was given funeral with full military honours and in 1932 the crew of HMS Hood, which was visiting the island, clubbed together to get a cross made to mark his grave. And there it stood until found by Mr Hill, nearly 90 years later.
Mr Hill did a lot of research into Mr Rees’s story and said it had been interesting. He was keen to contact Mr Rees’ family, but has had no luck yet.
n A small ceremony to reinstate the cross will take place at the cemetery at 11am on Saturday, on the 90th anniversary of his death.