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.