Two French tourists have died while snorkelling on the Great Barrier Reef.
The marine park's peak tourism body suggested both holidaymakers may have had heart attacks in a rare tragic coincidence before they were found floating near idyllic Michaelmas Cay.
The travellers, described as "elderly" by the cruise provider, had been on board the Passions of Paradise-owned catamaran as part of a group of 21 French tourists.
Passions of Paradise chief executive Scotty Garden said a third person also suffered a medical condition but survived.
Mr Garden expressed the company's sympathy for the deceased's family and friends, saying they had pre-existing medical conditions and were accompanied by a guide in the water.
Mr Garden said the company had stationed a lookout on the beach, another on the boat and two guides in the water at the sandy reef island, a little more than 40 kilometres north-east of Cairns.
"The Passions of Paradise lookout on the beach noticed a man floating in the watrer and pulled him to the sand cay where he performed CPR," he said in a statement.
"He was then assisted by a doctor on the sand cay."
After speaking to the company owner, Association of Marine Park Tourism Operators chief executive Col McKenzie said the woman was spotted face down in the water by the lookout on the boat.
"He grabbed her and dragged her onto the boat and commenced resuscitation of her but in both cases they were unsuccessful," he said.
Mr McKenzie said it was suspected they had both suffered heart failure, with the third person possibly suffering an anxiety attack.
"To have two people have heart attacks is just almost unheard of," he said.
"It may have been one after the other, it might have both occurred at the same time.
"The lookout on the beach rescued one. The lookout on the boat rescued another. Until we get the full details and people have been spoken to, we just don't know."
Mr McKenzie said Passions of Paradise was an "extremely good company", with an 80-year-old heart attack victim the only other fatality since it began operating in its 27-year history.
He said the latest deaths meant eight people had died on the reef this year, up from an average of five a year.
The Passions of Paradise website describes the area as a "world-famous bird sanctuary with a beautiful sandy beach, providing a rare opportunity to snorkel and dive straight off the beach in protected, calm waters".