It's 8am at the Crystalbrook Superyacht Marina in Port Douglas and business is brisk. Tour buses pull in and groups quickly assemble to board their boats for cruises out to the Great Barrier Reef for diving or snorkelling.

The marina is one of the main northern gateways to the reef and it is filled with multimillion-dollar boats. The reef is big business here. But the prosperity brought by hundreds of visitors each day masks a deep underlying worry about the future.

Tour operators rely on the reef staying healthy. But damage from rising sea temperatures, water pollution and a plague of coral-eating crown-of-thorns starfish has trimmed the number of areas of reef that remain in good condition for tourists to see. "The most dramatic change by far is just in the last two years," said Mr John Edmondson, who runs Wavelength reef tours with his wife, Jenny, from the Port Douglas marina.

The back-to-back bleaching episodes of 2016 and last year caused damage on a scale he said he has never seen before.

"Every year now there is this thing at the back of your mind, 'Will we get through the next season without it being too hot?,'" said Mr Edmondson, 52, a marine biologist. He and his 44-year-old wife have owned Wavelength for about five years and he started in marine tourism in 1992, when he became a dive instructor in the Whitsundays in the southern sector of the reef. He says he is still optimistic and he has seen signs of the reef starting to recover. But it needs time.

During a trip out to the reef, the Wavelength crew explain the impact of bleaching and climate change to visitors, a mix of Australians, Europeans and Americans. The aim was not to hide the threat but to educate.

"We want to take those people out and tell them the real story," Mr Edmondson said.

By doing so, it might very well be the best way to save the reef.

"You can still go out there and you can see some beautiful corals and you can learn about the reef and it's very hard not to fall in love with it and want to protect it," he said.

Despite the climate change threat, he said Wavelength is as busy as ever. "We don't envisage bleaching putting us out of business but we can see how it could."

The firm recently commissioned a A$3 million (S$2.9 million) catamaran to join the other three vessels in its fleet.

"This is the only thing we do and we're passionate about it."