The 49-year-old is believed to have been exploring a wreck in Scapa Flow.
The man, who has not been named, got into difficulties around 11am on Tuesday.
Stromness Lifeboat and Stromness Coatguard Rescue Team were tasked to the unconscious diver who was on a dive boat. Initial assistance was also given from local vessels in area.
The man, who was not from Orkney, was transported to Houton Pier on a vessel while the lifeboat crew conducted CPR.
They were met by police and ambulance. But the man was pronounced dead at the pier.
A police spokesman said: “inquiries are ongoing to establish the full circumstances and a report will be submitted to the Procurator Fiscal in due course.
Scapa Flow is one of the world’s most historic naval and dive sites. It was home to the British Grand Fleet during the First and Second World Wars.
