I was on a friend’s boat with a bunch of his freediving buddies. One of them used to train Navy SEALs in underwater combat. That’s where you hold your breath and beat the crap out of someone, surfacing only to breathe through a snorkel.
Most people don’t realize this, but humans are natural divers. We’re related to otters and dolphins and platypuses, by virtue of being mammals.
Underwater, most people feel an urge to breathe after about 30 seconds. This is caused by high CO2 levels. Keep holding, and the diaphragm will start to contract with the onset of hypoxia. Stick that out, and the body soon loses motor control. Then you black out.
Blacking out underwater sounds scary, but it’s really just the body’s way of shunting blood from the brain, because the brain consumes a lot of oxygen. Think of it as “sleep mode”. Blacking out doesn’t cause any long-term brain damage. My evidence is all anecdotal, so interpret as you see fit.
While blacked out, the laryngeal cords contract, blocking water intake. The body is safe. 4-6 minutes later, when oxygen is fully depleted, the body makes a terminal gasp. If you’re still submerged, that’s when you drown.
SEAL trainees dive underwater until black out, and are then brought back to the surface to wake up. This is repeated until underwater blackouts become familiar and comfortable.
The first step to underwater combat is to overcome a fear of drowning. You can’t be a good fighter unless you’re unafraid of death.