JOIN USShadowed by sharks and fighting severe dehydration, 48-year-old Penny Palfrey, from Townsville in Queensland, swam 108 km between two of the Cayman Islands, beating the previous mark of 101 km.
She spent 40 hours and 41 minutes in water, and was followed closely by four "curious" white tip sharks. Palfrey said she was "well aware" of the sharks, known for their aggression.
"One of the white tip sharks, it just shadowed me the entire night - I could see it sort of five feet away, may be more," she said.
"I had the (electronic) shark shield on, so it was just outside the range of the shark shield and it was just cruising underneath me all night long."
Steven Munatones, observer for the International Marathon Swimming Hall of Fame, confirmed the record. He described Palfrey's swim as "beyond incredible".
"She was weak and her arm strokes were just slicing through the water. She wasn't moving that fast but she just hung with it," he said.
Palfrey was the first woman to swim the Alenuihaha Channel from Hawaii to Maui and the first person to swim from Santa Barbara Island to Point Vincente off California.