A cruise ship passenger who was rescued from the Adriatic Sea 10 hours after falling overboard is in “excellent condition,” a doctor said Monday.

The 46-year-old British woman is “very healthy,” Irena Hristic, director of the Pula Clinic in Croatia, told reporters.

“Probably that's the only way she survived,” the doctor added. “She's nice lady. A healthy lady."

The rescued passenger, who identified herself to reporters as Kay, fell from the rear deck of the Norwegian Star as it sailed from Dubrovnik to Venice.

The crew of the ship turned the vessel around and used surveillance video to pinpoint where the woman entered the water — at about midnight on Saturday night, 60 miles offshore.

The exact circumstances of how she fell remained unclear.

However, Hristic said the relatively warm waters of the Adriatic Sea, between Italy and Croatia, likely prevented the onset of hypothermia.

“She doesn't have any bad injuries on her body,” the doctor said. “I'm also surprised that after 10 hours she is looking so good and that she's in … excellent condition."

As the woman stepped off a Croatian rescue boat Sunday, she told local broadcaster HRT that she fell from the cruise ship.

“I was in the water for 10 hours, so these wonderful guys rescued me,” she said, adding, “I am very lucky to be alive.”

The rescue boat captain, Lovro Oreskovic, described her as exhausted. "We were extremely happy for saving a human life," he said.

Lynne Cox, a long-distance swimmer and author of "Open Water Swimming Manual: An Expert's Survival Guide for Triathletes and Open Water Swimmers," said that being able to relax and gently tread water is crucial to surviving a long stretch in open water.

“If you are in going to do a long or cold swim, you need to feed along the way,” Cox said.

But perhaps the most important to element to surviving 10 hours is believing that you can, Cox said.

“Her resolve must be really substantial,” she said. “To be that far, to be alone, to be in the middle of the sea and keep hanging on there for 10 hours — she must have had a lot she wanted to live for.”

In a statement, Norwegian Cruise Line said: “We are very happy that the individual, who is a U.K. resident, is now safe and will soon be reunited with friends and family.”