The Guinness World Record for breath-holding belongs to Aleix Segura Vendrell, who managed 24 minutes and 3 seconds floating in a pool. How do Vendrell and others, like free divers and, er, David Blaine (see below) do it? Psychological training is obviously the first step, says Clayton Cowl, chair of preventive occupational and aerospace medicine at the Mayo Institute. But there's physiology at work too. From Smithsonian:
Olympic swimmers seem to be able to go great distances without breathing, but that is primarily due to aerobic conditioning, says Cowl. Those athletes are more efficient at getting oxygen into the tissue and extracting carbon dioxide. That allows them to breathe more effectively, and potentially, improve their breath holding.
Just being in the water may confer additional breath-holding ability. All mammals have what is known as a diving reflex. The involuntary reflex is most obvious—and pronounced—in aquatic mammals like whales and seals. But humans have this reflex, also. The purpose seems to be to conserve the oxygen that is naturally stored throughout the body, according to one study.
When a mammal dives into the water, the heart rate slows, and the capillaries of extremities like arms and legs—or flippers—constrict. Blood and oxygen is redirected towards the internal organs. The reflex helps diving animals override the need to breathe, which means they can stay underwater longer.
"What's the Longest You Can Hold Your Breath?" (Smithsonian)