TWO American divers who spent seven hours lost at sea on the Great Barrier Reef will today put their ordeal behind them and get back in the water.

The pair - a Pennsylvanian woman named Michelle and a California man named Babek, both aged in their 40s - were last night in "good spirits" back aboard their dive boat, the Cairns-based Spoil Sport.

Mike Ball, who operates the luxury 30m catamaran, said the pair were a "little shaken" but likely to continue with the $4500, seven-day dive expedition they started with 18 other guests and 11 crew on Thursday night.

"It's bloody good news," Mr Ball told The Weekend Post.

"If it had been a couple more hours they would have been in the drink overnight and who knows what would have happened.

"The odds of finding someone in the water at night tend to get quite slim."

They were spotted by a search helicopter in fading light about 5.10pm after drifting nearly four nautical miles north of their outer reef dive site.

The two experienced divers - Babek had logged about 70 dives, and Michelle 30 dives - failed to surface at the end of an hour-long dive on Ribbon Reef No 10, near Lizard Island about 10am.

The were pulled from the water by Lizard Island boat Fascination before being taken back to the Spoil Sport where a doctor and cardiologist, who were part of the expedition, gave them the all clear to continue their trip.

"They are two very lucky people," Cairns District Police Insp Brent Carter said.

"We had to throw everything into the search.

"Another hour and a half and it would've been dark and a different proposition. By then they would've been floating around for 16 to 18 hours and exposed to marine predators."

Cairns doctor Graham Simpson, a member of the South Pacific Underwater Medicine Society, said hypothermia would have been the main threat to the divers had they remained missing.

"Even in water of that temperature, they'd lose heat pretty quickly," he said.

Mr Ball said the dive site at Challenger Bay was neither particularly deep nor challenging.

The alarm was raised after the pair failed to return to the Spoil Sport as scheduled. Crew immediately started emergency procedures and alerted nearby vessels who joined in the search.

"At 11.40am we notified the Australian Marine Safety Authority and by 1.40pm a plane was on site and helicopters were on their way," Mr Ball said.

"We have done everything by the book."

Mr Ball said the pair had been given specific instructions on how to handle a "very safe" dive site.

Insp Carter said the rescue was testament to the region's emergency response capabilities.

Workplace Health and Safety will investigate how they drifted so far from the boat.

Yesterday's incident comes 11 years after Americans Thomas Joseph Lonergan, 34, and his wife Eileen Cassidy, 28 - both of whom worked for the Peace Corps - disappeared while diving on St Crispin Reef, near Lizard Island.

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