Award-winning American environmental photographer Gary Braasch died on Monday while snorkeling at the northern end of the Great Barrier Reef.
Braasch, 70, from Portland, Oregon, was snorkeling with a companion at the Australian Museum's Lizard Island Research Station, the museum said in a statement. He was documenting the effect of climate change on the reef.
The cause of death was not immediately clear. Queensland police had been notified and were investigating, the museum added.
A Nikon "Legend Behind the Lens" photographer, Braasch was an active climate change campaigner and had received many major awards and citations for his work.
The Great Barrier Reef, a World Heritage Site, is the world's largest living ecosystem, with thousands of multi-colored coral reefs stretching over 2,000 km (1,200 miles) off the northeast coast of Australia.
Parts of the reef face permanent destruction if the current El Nino, one of the strongest weather patterns in two decades, does not ease this month, scientists say.