Two French tourists in their 70s would have been in the water for less than 10 minutes at a popular Queensland snorkelling spot before suffering suspected heart attacks, which killed them, police say.

The man and woman were among a group of 21 elderly French nationals on a snorkelling tour at Michaelmas Cay north of Cairns with dive company Passions of Paradise on Wednesday.

Inspector Peter Mansfield said the 74-year-old woman entered the water around 10.45am and was spotted floating motionless a short time later.

She was then brought back on to the boat but couldn't be revived.

"While that was happening ... the 76-year-old man was observed also unconscious in the water," Insp Mansfield told reporters in Cairns.

"He was taken to shore and CPR attempted, but that was also unsuccessful.

He said the event unfolded very quickly and the pair hadn't been in the water for long.

"Certainly with the female person, it would only have been less than 10 minutes."

Police have confirmed the pair were not related and their next of kin were also on the boat, and are assisting with inquiries.

Association of Marine Park Tourism Operators executive director Col McKenzie said the pair suffered sudden medical issues.

"I just don't know what it was; however, from what I was told, it would lead you to believe that they probably had heart attacks," he told AAP.

"I do know that the woman was on some pretty severe medications as well."

Passions of Paradise chief executive officer Scotty Garden extended the company's sympathies to the family and friends of the tourists.

"They had pre-existing medical conditions and were accompanied by a guide when they were in the water," he said in a statement.

A third person in the group later suffered an undisclosed medical condition, but survived.

Mr McKenzie said that person was being monitored.

The company had taken more than 400,000 tourists to the reef since if began operating in 1989, Mr McKenzie added.

One other fatality had occurred during a tour in 1997, an 80-year-old who is believed to have died from a heart condition.