It was held on the 90th anniversary of the death Royal Marine Thomas Edwin Rees.
A parade marked the occasion, along with the Last Post bugle call, a minute’s silence and the laying of a poppy wreath.
Castel rector the Rev. Scott Lamb gave a short speech and led prayers.
Matt Gill, a sergeant with the Royal Marine Cadets, instigated the special event after he spotted the grave and its cross in a very poor condition.
He explained how he and his dad, Alan, organised for the grave and cross to be repaired.
‘The cross writing is in white, and the cross has now been oiled with linseed oil, so it’s a nice dark rich brown,’ said Matt.
‘The base around the grave is a treated pine, and my dad used to be a French polisher, so he stained it to get the colour as close to the cross as possible, and that’s also been treated with linseed oil to preserve it.
‘Underneath that we’ve put some Mypex down to stop the weeds coming through, and then the centre of the grave we filled with blue slate from B&Q.’
Mr Gill said the project had been a labour of love, and that everyone deserved to be remembered.
‘It felt like the right thing to do, and the great thing about the Marines is that it’s like a big family, even with the cadet forces. No matter where you are, if you’re a Royal Marine cadet it’s like a brotherhood, hopefully it’s going to be a sisterhood as well because we’re trying to recruit more girls.
‘My dad did dip his head this morning because I said that my next project I’d like to try is cleaning up the rest of the other graves at Fort George.’
The cross reads that Thomas Edwin Rees of the Royal Marines died of drowning aged 23 years on 19 June 1931.
An article from the time in the Guernsey Evening Press reports that he had spent the evening with shipmates drinking at the Brighton Hotel and the Channel Islands Hotel, but was considered relatively sober.
When the group went back to the White Rock to return to their ship, HMS Nelson, Mr Rees fell into the water.
Witnesses tried unsuccessfully to rescue him and he was later found drowned in the harbour.
Mr Gill wanted to thank everyone, including all the ex-servicemen and women, who turned out for the ceremony.
He is still trying to track down relatives of Mr Rees, to let them know that their ancestor’s final resting place is a tranquil one.