PORTLAND – Lynne Cox never imagined it would take more than two decades to line up a publisher for her memoir about the two record-breaking English Channel swims she completed as a teenager.

But the years spent writing, revising, switching agents and watching rejection slips pile up might have been a blessing in disguise. During that time, Cox achieved some of her most celebrated triumphs, including the first-ever swim across the Bering Strait and a mile-plus swim to the frozen shore of Antarctica.

Those swims, and others that took her to some of the world’s coldest, most remote and least hospitable waters, are detailed in “Swimming to Antarctica,” (Alfred A. Knopf, $24.95), an inspirational account of a personal journey that tests the limits of human endurance.

“This is really a book about dreams as much as it is about swims,” she said in an interview. “It’s about following your heart as much as it is about working hard, and it’s about just doing things when you don’t think you can and figuring out another way to do them.”

A Boston native, Cox, 47, learned to swim at the age of 3 at her grandparents’ camp on Snow Pond in central Maine. She spent most of her childhood in New Hampshire before her family moved to California to provide better swim training for Cox and her three siblings.

There, Cox recognized that her future in the sport was in swimming long distances in open water, not racing laps in a pool. She broke the men’s and women’s records when she swam the English Channel at age 15 and bettered her time a year later.

After a record-setting swim of California’s Catalina Channel, she went on to test her endurance in far-flung waterways from the Strait of Magellan and Cook Strait between New Zealand’s North and South islands to the shark-infested waters of the Cape of Good Hope and Siberia’s Lake Baikal.

As she got older, her goals evolved from trying to be the world’s fastest to swimming waters that had never been swum before. “The courses that are unplotted, the mountains that are unclimbed,” she said.

Because of her ability to endure extreme cold, Cox developed an interest in hypothermia and began working with scientists who were studying human survival under frigid conditions.

Cox believes her ability to endure such conditions are a measure of her training and genetics. Her blood vessels at the extremities tend to constrict in the cold, closing off blood flow and preventing cold blood from returning to her body core and brain.

Her proportion of fat to muscle is perfectly balanced to achieve neutral buoyancy, enabling her to swim with maximum efficiency and make her “at one with the water.”

Ever the purist, Cox doesn’t wear a wet suit – even during the 2002 swim to Antarctica, where the water temperature was 32 degrees and she was greeted by a flock of penguins when she reached shore.

“The swim really was about the human reach,” she explained. “It was about what humans are capable of doing and how far we can go. And if I wore a wet suit, what does that mean? What does that say? Pretty much nothing. Technology helped you get there. So it doesn’t appeal to me.”

But Cox is no daredevil, and she strives to minimize risk. Her exotic swims came after prolonged research into such factors as tides, currents and weather conditions. And before going to Antarctica, she underwent a series of fluoride treatments to help prevent her teeth from shattering in the extreme cold.

Still another goal Cox has pursued through her swims is to help promote international goodwill and bring people together. That thread was most apparent in the Cold War-era crossing of the Bering Strait, which opened that point along the U.S.-Soviet border for the first time in nearly a half-century.

It took 11 years of persistent requests before Cox obtained permission for the 2.7 mile swim in 1987 that ended in 38-degree waters. Former Soviet leader Mikhail Gorbachev hailed her achievement when he met with then-President Ronald Reagan; Cox returned to the Soviet Union, where she was greeted like a cosmonaut returning to Earth.

Other symbolic swims with the goal to ease world tensions were in the Gulf of Aqaba between Israel and Jordan and in the Beagle Channel between Chile and Argentina.

Cox has financed her swims largely on her own through work as a freelance writer and motivational lecturer, although she has picked up a few sponsors in recent years.

She maintains an arduous training schedule, combining daily swims when possible with workouts in the gym and long walks. She said she is stronger now than she was 15 or 20 years ago and is considering some new tests of her abilities as a swimmer. “But they’re risky things. I have to research them and see if I can manage those risks. It’s a long process.”

Cox says the challenges she confronts on her marathon swims are mental and spiritual as well as physical.

“If you don’t have a real strong purpose for why you’re doing it, then when you get tired and don’t think you can go much further, then it becomes less likely that you will,” she said.

During her stop in Portland on a nationwide book tour, she visited the waterfront and looked toward Peaks Island, about 2.4 miles away. With air temperature hovering around 20 degrees and the water at 38, could anyone swim there under such conditions?

“I think if it was a matter of survival, I could do it,” she said. “But I wouldn’t want to do it now.”