Richard Neely and Allyson Dalton's battle to stay alive for 19 hours in the tropical waters of Australia's Great Barrier Reef ensured them a place in the record books. Their tale of survival against the odds had all the elements of a spell-binding narrative: an idyllic holiday that turned into a nightmare, panicked hours spent desperate and stranded in the deep, shark-infested waters, and cries for help that went unheard. It ended, of course, in dramatic rescue, relief and a story that captured headlines all over the world.

From start to finish, the media cried, it was a tale of stupendous courage, almost superhuman stamina and intense drama. But, as the dust settles on the reports of their awe-inspiring struggle, those who originally praised the couple are starting to ask questions. And, unlike four days ago, when it was the operators of the boat and the pair's fellow divers who were under fire for alleged negligence, now Mr Neely and Ms Dalton are in the spotlight.

Scuba experts have wondered why the couple went diving in tropical 23C waters wearing 9mm hooded wetsuits when everyone else on their boat wore a 2mm stinger suit. It has also been queried as to why they took an expensive shark-repellent device when all they were planning was a casual afternoon's diving.

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Those onboard have also claimed the couple ignored safety instructions about leaving the lagoon they were diving in and that the distance they surfaced from the boat was much greater than was first suggested. While Neely and Dalton say it was 200m, it has been said that it could have been nearer 2km.

And, perhaps most curiously, it is being asked why the couple did not inflate their orange surface marker buoy immediately upon surfacing as they had done on five previous dives during the same trip. Instead they waited until nightfall.

These questions, among others, have caused holes to emerge in the divers' apparently watertight story. Yesterday, as they flew to America with a public relations guru to sell their story, which has already earned them anywhere between £5,000 and £500,000 in Britain, their version of events was coming under increasing critical scrutiny.

Mr Neely, who is 38 and originally from Norfolk, had boarded the Pacific Star on Wednesday evening with his American girlfriend, Allyson, and 18 other passengers and four crew members. That evening the couple, both experienced divers, sat and chatted with other passengers.

Rebecca Sharkey, 24, from Liverpool, was one of the others on board. "Everyone was drinking and getting along and treating it like a normal holiday but you could tell straight away that Rick and Ally were more interested in the diving than the holiday aspect of the trip," she said. "They wouldn't stay up as late as the rest of us and were quite sure about exactly where and when they wanted to dive and what they wanted to see."

On Thursday afternoon the group took their first dips and retired to bed early ahead of Friday's schedule. Michael Paton, 18, from Anglesey in north Wales, had lunch with the couple aboard the boat before the Friday afternoon dive – the dive that would see them lost at sea for 19 hours.

"They were a really friendly couple and were telling us about their diving experience," he said. "My friends and I had a lot of respect for them because they are seriously well-qualified divers. He is a master scuba diver and she is a dive master; they were probably better qualified than the instructors on the boat.

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"Over lunch we spoke about diving and they told us about the shark-repellent device they had brought. It is a seriously expensive piece of kit; I reckon it would cost about £1,000. I do remember thinking it was a bit strange to have such extensive equipment for a dive around a small lagoon, but then again if I had one I'd probably have taken it out with me too."

After that lunch the passengers split up into groups of six to go diving in Gary's Lagoon. Ms Sharkey was part of the group that entered the water with Mr Neely and Ms Dalton. She said: "I did notice that their suits were different to everyone else's. We were wearing 2mm stinger suits simply to protect us from the jellyfish, but they were wearing 7mm wetsuits on top of the 2mm stinger suits, giving them 9mm of protection. You normally only need something for that if you're diving in very cold water or spending a lot of time in the water.

"But I didn't find it too strange because they had brought their own kit and I had rented mine. If I had my own suit and it was as good as the suits they had, I'd have worn my own too."

Ms Sharkey says the group were given instructions not to leave the lagoon and told to resurface immediately if they did so. But she says she believes the couple ignored these rules. "After we had been given the briefing they were talking about going to find manta rays and eagle rays. You can't see things like that inside the lagoon so it seemed quite clear they were going to leave the lagoon, but it wasn't really a big deal. They are both so experienced that I assumed that even if they did leave, they would know what they were doing and be back in time. As soon as we got under water I looked around and they were gone. I assumed they'd gone out of the lagoon, which was a direct violation of what we had been instructed to do. They were definitely reckless."

Following the dive, Mr Neely and Ms Dalton claimed that they were only 200m from the boat when they resurfaced and, despite their shouts and whistles, were not spotted by the crew. But Ms Sharkey finds that hard to believe. "At one point my dive buddy and I wandered out of the lagoon by accident and as soon as we realised we surfaced and waved for help. We were about 150m away from the boat and simply by waving we were spotted immediately and the smaller boat came out to get us.

"If Rick and Ally were 200m away they'd have been just a short distance behind us and easily visible to both myself and the crew. But I didn't see them and the crew obviously didn't so I don't see how they could have been just 200m away."

The final question is why the couple did not use their orange safety buoy to signal their position in the water. The balloon is visible for one nautical mile and would easily have been spotted if Mr Neely and Ms Dalton were in the vicinity of the boat. In the original Sunday Mirror interview, Mr Neely said he was waving the buoy from the start, but Australian newspapers have reported that the couple told police they did not inflate this until it got dark.

Ms Sharkey added: "Every other dive they had done, even if they had surfaced just 20m from the boat, they were inflating their balloon; it was just good practice. I can't understand why they would have waited so long this time."

Her sentiments were echoed by the boat operators, Ozsail, who released a statement simply saying: "Allyson and Richard did not remain on the dive site. Allyson and Richard did not follow the clear instructions of the dive instructor. Allyson and Richard did not surface immediately upon leaving Gary's Lagoon." Mr Neely and Ms Dalton were found on Saturday morning, 14km from where they first entered the water. The search for them involved seven helicopters, three aircraft and many boats, and is estimated to have cost up to £200,000. Since then the couple have, in the numerous interviews they have been paid for, denied any wrongdoing and even said that they were disappointed in the reaction from the boat's crew and passengers.

Mr Neely has been quoted as saying: "We don't consider that we drifted away from the dive site. We were on the dive site for the entire time of our allocated dive time. I would consider that our (dive) plan went as expected." However, in another interview he said: "He (the boat's skipper) definitely did not say we should surface if we left the lagoon."

On the subject of the crew and passengers' efforts to find them, Ms Dalton said: "Each moment ticked by and we were drifting further and further away and we realised that they were not coming." Mr Neely added: "They may have been looking for us in the lagoon, but I don't think they were looking for us in the right direction."

It is these comments which have irked fellow passengers. "When they didn't return as scheduled we started worrying and everyone was on top of the boat using binoculars to try to look for them," said Ms Sharkey. "We stayed on the boat all night and I know for a fact that I didn't get a wink of sleep that night. I'm pretty sure most others on board didn't either. To have them say we didn't do enough was quite hurtful. They are now selling their story on television and to the newspapers and it hurts that they are being nasty about us when we put in so much effort for absolutely no gain of our own. We just tried to help."