Visitors to Australia's Great Barrier Reef will be forced to wear life vests if they are deemed to be "at-risk" swimmers, a new safety code says.
The rules are a response the deaths of 10 people - including at least four foreign tourists - on the reef in 2016.
Under the Queensland state code, tourism operators will be required to identify swimmers who are potentially vulnerable before they enter the water.
They may include elderly, inexperienced or unfit swimmers, authorities said.
The Great Barrier Reef is one of Australia's most popular tourist attractions, composed of thousands of individual reefs and hundreds of islands along the Queensland coast.
Recreational snorkelling usually takes place from boats, which transport tourists to reef and island spots.
Two French tourists, a British scuba diver and a Japanese snorkeller were among those who died in 2016.
Operators will now be required to pair some swimmers with a partner, and carry defibrillators on their vessels.
"This finalised code will ensure visitors who are diving and snorkelling in our state feel it is being done in the safest manner possible," said Grace Grace, a state government minister.
More people had died while snorkelling than diving, authorities said. Common risk factors included poor fitness and inexperience with swimming.
The risk of fatality also increased with age and pre-existing medical conditions, the government said.