At times they were struggling to remain afloat amid 25ft waves, and visibility was reduced to just a few yards as the rain lashed down. As the light faded, the Byrnes feared their hopes of a rescue had gone with it.
"We thought we were done for, that we'd perish in the water and our bodies would never be found," said Julie.
"We saw helicopters overhead and although we yelled and screamed they couldn't see us."
Their ordeal bears remarkable similarities to the 2003 film Open Water, which chronicles the desperate struggle of a couple who find their boat has vanished after surfacing from a scuba-diving trip in the Caribbean. The movie was based on the real-life story of American tourists Tom and Eileen Lonegran who disappeared off Australia's Great Barrier Reef in 1998 and were never found.
Fortunately, the Byrnes did not share the fate of the film's lead characters - one drowns and the other is killed by a shark - although just ten minutes before they were finally spotted, Jeff felt two thuds against his legs.
"I got bumped twice quite hard and it had never happened all throughout the seven hours before," he said. 'It was my left leg first and then my right leg. I had my mask but I thought, 'I don't want to look down there if there are sharks'."
The couple, from Rockcliffe, Cumbria, who have two daughters aged 25 and 22, had been looking forward to the ten-day trip in June. Staying in the luxury resort of Veranda Paul in Grand Gaube, they had booked a series of dives through their hotel with private firm DiveSail Travel.
After a successful excursion on their third day, they booked another for the following morning.
That day, the couple were among a group of tourists taken out by speedboat to Gunner's Quoin island, off the northern coast of Mauritius.
A French dive master jumped into the water alongside the Byrnes, a German woman, Patricia Veccio, and a 51-year-old British man, Jeffrey Tibbles, and began what was supposed to be the first of two dives that day. Yet, without their knowledge, the boat left the spot to drop off other divers around the cove.
"If I'd known the boat was going to leave us, I would absolutely never had gone in the water. It's a diver's worst nightmare," said Jeff, a site manager and veteran of more than 500 dives.
After 30 minutes underwater, visibility became poor and the dive master signalled to the group to return to the surface. But to their astonishment, their boat had vanished.
Julie said: "Panic set in immediately. The dive leader told everyone to remain calm and started blowing his whistle, saying the boat would hear it and come back. But we quickly realised no one could hear us and the boat wasn't coming back."
At that point, the couple say they were just 50 yards from the shore of Gunner's Quoin. But, incredibly, the dive master feared they would be washed into nearby rocks, so he ordered them to swim away from the island.
"The instructor was yelling at everyone to swim further out to sea," Julie said.
"It was like the instructor had no safety training. He had no radio, no SOS equipment, no way of calling for help.
"We then got caught up in a riptide and were carried out by the current."
Her husband added: "The skies went very grey and it started to rain. It got so dark that we could barely see each other. At that point, even if anything came it was never going to find us. That was about my lowest point."
Meanwhile, a huge search operation involving a helicopter and 20 vessels had been launched.
As the hours dragged on, Jeff began to fear the worst. "Not all five of us would have made it through the night, definitely not," he said.
"Three of us were quite strong, two of us not so strong. The German girl got really sick with the heavy seas... she more or less gave in. She was vomiting all the time. She went very quiet."
Julie, used to dealing with emergencies in her day job as an ambulance support centre operator, said: "Jeff told me to stay strong and have hope. But when the helicopter passed us by, I was on the verge of breaking down.
"I told Jeff I loved him and he said he loved me."
At 5.15pm, they were finally spotted - the crew of a pleasure boat searching for them saw Jeff's 5ft inflatable surface marker. The group was severely dehydrated and sunburnt - but alive.
"We were all in tears, we were elated," said Julie.
"We all just got on the boat and we were hugging and kissing. Even the two lads who rescued us were in tears. We're just so grateful to everyone who joined the hunt."
An investigation by the Mauritius Scuba Diving Association (MSDA) found that the skipper and the dive master were negligent.
An MSDA spokesman said the Mauritius Tourism Authority had temporarily suspended the licence of DiveSail Consultants Ltd, pending a decision on whether it will be revoked. Last night the authority did not respond to requests for comment.
Stephane de Senneville, director of DiveSail Travel, which contracts out the scuba arm of its business to a third-party firm, DiveSail Consultants Ltd, said: 'The mistake was the decision made by the dive master... who chose to swim away from the protection of the cove and into sharp currents which dragged them out to sea.
"Everyone came out alive and no one was hurt - the end result was positive."
But for Jeff and Julie - who say they have yet to receive a formal apology and merely had their fee for the day's diving trip waived - the company's words provide little solace.
Julie said she is suffering post- traumatic stress disorder.
Jeff added: "I'm still adamant I would have survived and got three of us out of the situation we were in, but who knows. A shark could have come and taken someone's leg off and it could have been a different story all together."
