They entered the water together as newlyweds. Only Gabe Watson came back up.

The morning of 22 October 2003, was mild and perfect for diving when Tina Watson slipped into her wetsuit for what would be the last time.

Just 11 days earlier, she had been Christina Mae Thomas, walking down the aisle in Alabama towards the man she'd fallen in love with at university — David Gabriel "Gabe" Watson.

Tina was on her dream honeymoon on Australia's Great Barrier Reef, about to explore the historic SS Yongala shipwreck off Townsville with her experienced diver husband by her side.

Tina and Gabe Watson had been married for 11 days. Image: Supplied.

After some toast and cereal for breakfast, a small group of divers entered the azure blue waters.

Within 10 minutes, Tina would be brought to the surface without a pulse by instructor Wade Singleton, who had found the 26-year-old unconscious on the sea floor, 29 metres below the surface.

She was captured forever in the background of a stranger's holiday photograph, a haunting image that would become the centrepiece of a mystery spanning two countries and five years of investigations.

Tina couldn't be revived.

While her cause of death was ruled to be drowning, suspicions about her husband began to swirl.

Questions arose as to how an experienced rescue diver would breach such fundamental dive rules in leaving his wife alone, as she was struggling.

Lindsay Simpson, co-author of the book Honeymoon Dive, told Mamamia's True Crime Conversation podcast things started "not adding up" after Gabe Watson returned to the boat, without Tina.

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"Gabe popped back onto Spoilsport (the dive boat) and that comment was made by at least one of the passengers, like, 'if that was you over there, honey, I'd be there with bells on', you know, working out what was going on," Simpson said, explaining Tina was initially taken to a separate dive boat, the Jazz II.

"(For) quite some time he was wandering around Spoilsport, saying, 'oh, you know, something's happened to Tina.' And also, which I thought was interesting, telling every man and his dog what he thought had happened, including the chef.

"He wasn't behaving normally, and he wasn't going to Tina's side, definitely."

Tina and Gabe Watson travelled to Queensland via Sydney. Image: Supplied.

According to Simpson, Gabe's behaviour was so strange, other women on the Spoilsport noticed it.

"Their wives were talking about how strange the way he was reacting, that there were things that didn't add up, like he hadn't gone over to the boat to be with her, that he seemed to be, I mean, he was playing cards on the way back to Townsville when her body was lying in cabin 14 on the boat," Simpson said.

"Very strange behaviour."

The stranger who captured Tina's last moments would prove to be an invaluable witness to the events. His name was Gary Stempler, and he was an emergency doctor.

"You couldn't get a better, in my view, expert witness, than that … what he saw was this person lying on her back, and he said, already exhibiting signs that she was dying," Simpson said, adding that prior to the photo, Stempler described seeing what he thought was a bear hug between Tina and Gabe.

"That bear hug was interesting, because I think, and I think I'm right in saying this, that Gabe Watson denied he'd gone anywhere near Tina in any of the 16 different versions he gave about what happened … And he thought that he was the doctor, thought Gabe was helping her."

After being formally charged with his wife's murder by Queensland Police, Watson pleaded guilty to the lesser charge of manslaughter in June 2009.

Coroner David Glasgow alleged Watson held his wife underwater, turned the valve off on her air cylinder until she suffocated, switched the air back on, and let her sink to her death.

Watson would serve 12 months behind bars before being deported to face fresh murder charges in Alabama. In February 2012, he was acquitted of these charges.

Simpson noted Tina, who had heart surgery to correct an irregular heartbeat two years before her death, was a "panicked" and inexperienced diver and had refused offers of an orientation dive on Spoilsport.

"We went back to that quarry where she learned to dive," she said, explaining that she and her co-author met the instructor who trained Tina.

"He described how she panicked. I think it was 10 meters down, and we were able to really feel this pressure. And I still remember him saying, when he said to her, 'you shouldn't be diving. You're not able to do this. You're really the most panicked diver I've ever seen.'

"When they were on the boat, and she was asked about how many dives she done, she was very inexperienced."

Feature image: Queensland Police.