The staff of a Great Barrier Reef tour operator did all they could to save two French tourists who died after suffering heart attacks while snorkelling together at Michaelmas Cay off Cairns, the company's chief executive has said.

Key points: Two French tourists who died were a man and woman, aged 76 and 74, and unrelated

Two French tourists who died were a man and woman, aged 76 and 74, and unrelated Pair were among a group of 21 elderly people "with pre-existing medical conditions"

Pair were among a group of 21 elderly people "with pre-existing medical conditions" Police said CPR was administered amid what was a "very unfortunate event"

The man and woman, aged 76 and 74, were accompanied by a guide when they were in the water, Passions of Paradise chief executive Scott Garden said.

The pair, who were unrelated, were among a French group of 21 elderly people "with pre-existing medical conditions" on a catamaran tour of the reef, he said.

Queensland Police Inspector Peter Mansfield said onlookers were first to notice the woman, a short time after she entered the water about 10:45am, floating face down and not moving.

She was brought back to the boat and CPR was administered, without success.

About the same time, Inspector Mansfield said, the man was spotted unconscious in the water.

He was taken to the beach at Michaelmas Cay and CPR and defibrillation were attempted, but he too could not be revived.

Passions of Paradise has been operating on the reef for nearly 30 years. ( Supplied: Passions of Paradise )

Inspector Mansfield said the deaths were a "very unfortunate event" and that both people had next of kin on board the boat at the time.

He said police were investigating as part of the "coronial process" and would look at any first practices the company had in place.

Workplace Health and Safety would conduct their own inquiry, he said.

Protocols followed: Passions of Paradise

Mr Garden said the company would conduct a review, but he was confident protocols were followed.

"We did what we could. It was a credit to our staff, and we also had a neighbouring doctor on another boat that offered assistance as well, so we did everything that we could," he said.

He added: "We've extended sympathy, obviously, to the family and friends of the two elderly French people today. And also sympathy out to our crew that have had to deal with the situation at hand."

The company said it had a lookout on the beach, another on the boat and two snorkel guides in the water at the time of the deaths.

Col McKenzie from the Association of Marine Park Tourism Operators described the two deaths as a "rare tragedy", adding Passions of Paradise was a well-respected tour operator.

"You can image how shattered they are," he said.

"Just absolutely stunning event, a tragedy for everyone involved.

"It might just simply be a coincidence, but we may well find out that one person saw the other person being rescued and that caused the onset of the second emergency.

"We just don't know, it's pure speculation at this time."

Rescue helicopter unavailable to help

Mr McKenzie said the Cairns-based rescue helicopter wasn't available to help with emergency.

"We've been really concerned about the lack of the rescue helicopter over the last few months," he said.

"The latest fatalities, the helicopter hasn't been available. We have no idea why they're not providing any support.

"We just get told the helicopter's not available."