For the Lonergans, the trouble was that no one noticed they were missing on the trip back to shore either. Two days passed before anyone realised that the American couple from Baton Rouge, Louisiana, had been left behind in the open sea almost 40 nautical miles from shore.
In the eight months since they vanished, the mystery of what happened to the couple has deepened and theories have abounded. Some of their diving gear has washed ashore, intact, on the north Queensland coast. Eileen Lonergan's diary has been found, in which she wrote that her husband had a "death wish".
Was it simply an appalling mistake that they were left behind, to drown or be eaten by sharks? Or did they engineer their own disappearance - and, if so, why?
This week, an inquest opened in Cairns, Queensland. Johnny and Kathy Haines, Eileen Lonergan's parents, have flown in to attend.
With no body or body parts to prove that their daughter and her husband are dead, they must deal with speculation that they could still be alive.
Police have received at least 25 reports that the couple have been seen more than 1,000 miles away, in locations stretching from the outback of New South Wales to Darwin in the Northern Territory.
Thomas Lonergan, 34, and Eileen, 28, who were on holiday in Port Douglas, Queensland, were experienced scuba divers. Richard Triggs, a fellow passenger on the Outer Edge, told the inquest that, after the third and final dive of the day, the couple had told a diving instructor that they would "go off and do their own thing".
The boat returned to Port Douglas at about 3pm. But, police told the inquest, no one checked if all 26 passengers were on board.
Two days later Jack Nairn, the Outer Edge's owner and skipper, found the Lonergans dive bag on the boat.
He told police: "I looked in the bag and thought, `Jesus Christ, it's got a wallet and papers in it'."
Police and the Australian navy launched a sea and air search lasting several days, but there was no trace.
Then in early February, the Lonergans' scuba vests were found on a beach near Cooktown, about 100 miles north of Port Douglas. In June, a diving slate was found elsewhere with a message apparently scrawled by one of the Lonergans. "We have been abandoned ... by M.V. Outer Edge," it said. "Help!".
If the Lonergans did stage their "disappearance", then save themselves, they would have had to swim almost four miles to the nearest pontoon, at Agincourt Reef. But then they would have been marooned.
At the inquest, counsel for the Outer Edge's owners suggested that Thomas Lonergan could have murdered his wife, then committed suicide. Counsel for the couple's families said the idea was "outrageous", "disgusting" and "slanderous".
The most grim explanation has so far come from Ben Cropp, a veteran Australian scuba diver who lives in Port Douglas and knows the Barrier Reef intimately.
He said he thought the Lonergans were eaten by tiger sharks, probably soon after the Outer Edge departed. "Tigers are very cautious sharks," he told the court. "They just circle and watch. They may do this for an hour before moving closer and may follow you for another hour before they take that first bite, and then you don't have a hope."
But if the sharks did eat the Lonergans, how does this explain the discovery of their buoyancy vests, with no signs of blood, teeth marks or tearing, on the Cooktown beach?
Whatever the real explanation, the "left behind" theory seems to have taken precedence at the inquest. Noel Nunan, the coroner, said he anticipated that counsel for the police and the Lonergans' families would seek the laying of "charges of manslaughter based on criminal negligence". Whatever the outcome, speculation about what happened to the Lonergans is unlikely to fade away.Reuse content