Sara Burnett had initially planned to spend only a few weeks practicing at Blue Element Freediving, a training center in the Caribbean island nation of Dominica. The area is popular among free divers, who plunge deep into open water without a scuba tank, hitting depths that would make a civilian’s ears pop upon contemplation. The water is so warm that a diver, squeezed into a thick wet suit, has the not unpleasant sensation of being cooked sous vide.Burnett, who’s 41, grew up largely in Plano but has rarely been landlocked since: She now lives in St. Lucia, an island not far from Dominica. She started free diving in July 2023, less than a year prior to her visit to Blue Element, and was already reaching…