Police and other emergency services had responded to reports of a man in difficulty at about 13:45 on Saturday.
The incident took place as a yacht racing event was being held in the area.
The man was brought ashore but died at the scene. Police said no further details would be released until he has been formally identified.
A spokesman said there were no apparent suspicious circumstances.
A report will be submitted to the procurator fiscal.

Police now believe that the 26-year-old man from Rochdale took a crystallised and unrefined version of the drug Spice.
Ch Supt Neil Evans, described the development as "extremely worrying".
Ten people were admitted to hospital after taking the drug over the weekend, with four kept in intensive care.
A police spokesman said they were hospitalised after taking what was thought to be a new form of MDMA, known as "pink champagne".
But he added: "It is now believed that this substance was in fact a synthetic cannabinoid, like 'Spice', however it was sold as MDMA."
Ch Supt Neil Evans said: "This is the first time we have seen 'Spice' in crystallised, and much more potent and unrefined form and it's now being mistakenly taken as MDMA."
He added: "Anyone thinking of taking MDMA must seriously consider what they are doing.
"You may feel that you know and accept the risks of taking MDMA, but you never know what you are actually taking and the reaction your body could have to it."
A 26-year-old man, held on suspicion of supplying Class A drugs, was released while inquiries continue.

The Waking the Dragon tower was granted permission by Wrexham council back in February 2011.
Businessman Simon Wingett is trying to raise £2.5m to build the bronze sculpture, cultural centre and gardens at Chirk Park.
Councillors are expected to grant the extension at a meeting on Tuesday.
The 23.5m (77ft)-high dragon with wings spanning 57m (187ft) would stand on top of a 41.5m (147ft) tower at the former colliery site.
Mr Wingett has said the sculpture will rival other well-known landmarks such as the Angel of the North, but time is running out on planning permission for the project granted back in 2011.
He hopes to raise £1m for the visitor attraction from the public, with the rest of the cash raised through loans from Finance Wales and grants from the Welsh Government.
In a report to the council's planning committee, officers say the development would be a "significant tourism draw to the area".
Councillors are recommended to approve the extension allowing Mr Wingett five more years to commence building at the site.
A 69-year-old man has died after his boat capsized in the Findhorn Bay area of Moray.
[NEXT_CONCEPT]
A man thought to have been killed by the drug MDMA had taken a "much more potent version" of Spice, Greater Manchester Police has said.
[NEXT_CONCEPT]
A man hoping to build an 224ft-high Welsh dragon tower on the English border will ask councillors for an extra five years to start the project.