Tanya Streeter set a record on Tuesday in a new category of free diving, descending on a single breath to 35m and surfacing without the aid of fins.
She wore only a body suit for the dive in the "constant weight without fins" category, breaststroking down and up on her own power.
Hers was the first record in the category, which was just recognized in January by the International Association for the Development of Apnea, one of the sport's governing bodies.
After examining underwater video footage, judges confirmed Streeter had reached 35m and surfaced after 1 minute, 44 seconds.
The dive off the Turks and Caicos Islands marked the ninth time Streeter has set a free diving world record.
While several of those marks have since fallen, she currently holds the world records in five categories.
On Monday, she set a world record in the "variable ballast" category by descending on a single breath to 122m, using a weighted sled to descend and surfacing using fins.
On Tuesday, there were heavy rains and the water was choppy off the island of Providenciales in the British Caribbean territory, with waves up to 1m.
Before descending, Streeter said: "I feel a little nervous because of the rough seas."
After surfacing, she stood in the rocking boat and said: "Horrible conditions, but I'm glad I've done it."
The 30-year-old native of the Cayman Islands holds both US and British citizenship and lives in Austin, Texas.
She and other competitors say they take many safety precautions and are well aware of the potential dangers of descending on sheer lung power.
Another world-record holder, 28-year-old Audrey Mestre, died in October after plunging into deep waters off the Dominican Republic trying to break a "no-limits" world record of 159.5m set by her husband.