"The visibility has been reduced considerably, so when we take people snorkelling and diving, they ask 'how come the water's so dirty?'," says Al Grundy, whose tour company is based in the Whitsunday Islands.
Port development - intended to keep pace with rapidly increasing production in Queensland's coalfields - is only one of the pressures on the reef, which is also affected by storm damage, agricultural pollution, infestation of crown of thorns starfish (which prey on hard coral) and coral bleaching, caused by warmer waters.
But given those other factors, increased industrial activity is the last thing the reef needs, say critics.
"We don't deny there are serious existing environmental problems," says Felicity Wishart, director of the Australian Marine Conservation Society's reef campaign.
"But these are new, emerging ones that will make matters worse."
Although the World Heritage Committee has deferred until next year a decision on whether to list the reef as "in danger", it will consider in Doha a Unesco report deploring the federal Government's decision to allow 3 million tonnes of spoil to be dumped in the reef's waters during construction of new terminals at Abbot Point, north of Bowen.
As well as clouding the water, sediment, which can travel a long way, deprives coral and seagrass of the light they need to grow. An estimated 180ha of seagrass beds - the main diet of endangered dugongs (sea cows) and turtles - will be lost as a result of Abbot Point's expansion into the world's largest coal export terminal.
And although Campbell Newman's government claims dumping will take place 40km from the nearest coral, Grundy says one of his dive sites - Holbourne Island, described on a government website as "a major nesting area for turtles and birds ... [with] fringing reef and secluded beaches" - is just 8km away.
The tourism industry, he stresses, is not anti-development, but is advocating a pause while scientists try to establish the reasons for the swift deterioration in water quality.
"The reef is already in poor health and declining, because of all the activities we've been up to for the last 200 years," he says.
"So we have to try to halt and reverse that."
Grundy also questions why dredge spoil can't be dumped on land, or further out to sea - both more expensive options, but posing less of a threat to the reef, which has lost half its coral over the past 27 years.
"This is not only our livelihood. We're the custodians of this amazing reef, and we need to do everything we can to ensure its survival."
Australia expects to more than triple coal exports by 2030, with Queensland supplying much of that increased raw material. But opponents question the growth projections and the need for infrastructure expansion, noting that existing ports are operating at only 65 per cent capacity.
Larissa Waters, a Queensland Senator and environment spokeswoman for the Greens, condemned the ports strategy.
"It won't apply to any of the damaging dredging already applied for ... This is atrocious news for the Great Barrier Reef."
