(AP Photo/Dave Martin)

BIRMINGHAM, Ala. - A doctor testified Thursday he saw a U.S. man with his arms around his newlywed wife moments before she sank to the ocean's bottom and drowned during a honeymoon dive in Australia.

Stanley Stutz, an emergency room physician, said he believed at the time eight years ago that Gabe Watson was trying to save wife Tina Thomas Watson after she became distressed underwater. Prosecutors, however, contend he witnessed the woman's murder.

Jurors' perceptions of the doctor's story could be vital to the outcome of Watson's capital murder trial.

Watson served 18 months in prison in Australia after pleading guilty to a manslaughter charge in his wife's death. The charge involved negligence, not premeditated murder like the current charge.

Stutz said he was in the water with about three dozen other divers when he looked down and saw Tina Watson floating on her back with her arms extended, moving slowly in the water.

"I was close enough to see her face," Stutz said. The woman seemed distressed but wasn't thrashing and moved like she "had no energy," he said.

A man Stutz said he now knows was Watson swam to the woman, extended his arms under her armpits and lingered momentarily, the doctor said.

"Then they split apart. After, he went to the surface. She sank," Stutz said. The woman was clearly alive before the encounter, but she was dying afterward, he said.

Prosecutors contend Stutz saw Watson during the final scene of a murderous act. They claim Watson turned off his wife's oxygen and let her lose consciousness or become distressed before approaching her again, turning on her air supply to cover up the murder, and letting her sink.

Yet Stutz said he didn't think anything sinister had happened at first.

"I thought Gabe was trying to save her," Stutz said. "I was surprised when I got an email that it was a murder trial."

Evidence showed Watson, 34, was certified as a rescue diver in 1999, about four years before his wife drowned in October 2003. She was a novice diver.

Watson has remarried and was accompanied to court by his second wife.

The defence claims Tina Watson panicked during the dive and knocked off her husband's mask and air supply, forcing him to surface for help because the current had carried her away from him.

A master diver who was on the diving trip, Kenneth Snyder, said he and his wife talked with a bubbly Tina Watson and her husband on their dive boat before divers went in the water. Later, he said, he saw Watson board the boat alone.

"I asked Gabe where was Tina at. He said, 'She didn't come up,'" Snyder said. Snyder said he soon saw Tina Watson's body being taken out of the water and placed on the deck of another boat.

Watson didn't try to get to the second boat while others were trying to revive his wife, Snyder said. Snyder said he asked Watson what had happened underwater and immediately disbelieved the man's claim that his wife had panicked and knocked off his gear.

Earlier, Snyder said a rescuer only must press a single button on a distressed diver's gear to fill a bladder with air and send the other person to the surface.

"It will save their life?" asked prosecutor Don Valeska, an assistant state attorney general.

"Absolutely," said Snyder.

Prosecutors argue that Watson killed the woman hoping to collect some $210,000 in insurance benefits.