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'Sea Nomads' Are First Known Humans Genetically Adapted To Diving (nationalgeographic.com)
schwit1 shares a report from National Geographic: Most people can hold their breath underwater for a few seconds, some for a few minutes. But a group of people called the Bajau takes free diving to the extreme, staying underwater for as long as 13 minutes at depths of around 200 feet. These nomadic people live in waters winding through the Philippines, Malaysia, and Indonesia, where they dive to hunt for fish or search for natural elements that can be used in crafts. Now, a study in the journal Cell offers the first clues that a DNA mutation for larger spleens gives the Bajau a genetic advantage for life in the deep.
From TFA: When you go hold your breath and are surrounded by water you have a bunch of physiological responses that happen automatically. One major one is that your spleen contracts delivering more red blood cells to your arteries. Since they have larger spleens, there's a larger reservoir of red blood cells ready to get pushed out when necessary.
Hyperventilating is dangerous and is specifically advised against during freediving courses. The reason is twofold.
First, our urge to breathe is controlled by high CO2, not low O2, and hyperventilation removes CO2 from the blood without adding much O2 (it's already at 97+% of the possible maximum by default). Three deep breaths is the golden middle that fully replaces CO2 with O2 in lungs without affecting the blood composition too much. So those who hyperventilate do not, actually, get more time until a blackout, they just get "more comfortable" time, and "more comfortable" to a not really predictable degree. I.e. useless to really say "enough" to oneself.
Second, red blood cells are not willing to give away the oxygen they are carrying if there is not enough CO2.