I think there may be some very interesting physics involved.
Let's start by thinking about what happens when an aircraft at high altitude loses cabin pressure.
Just prior to decompression, the passengers are all breathing normally. They inhale a mixture of relatively inert nitrogen and about 21 percent oxygen. When exhaled, the air still contains about 16 percent oxygen.
Each breath only exchanges a small fraction of the total lung volume during breathing at rest. There is quite a bit of air with perhaps 18 or 19 percent oxygen available in the lungs.
Imagine now that a person intentionally holds their breath. The amount of oxygen in the lungs slowly falls as it is carried away by the blood and is replaced by carbon dioxide. For perhaps several minutes, the amount of oxygen in the blood is relatively unaffected because of that available oxygen already in the lungs.
Now lets imagine that person on an airplane has just experienced an unexpected decompression. The environment is suddenly at one-third atmospheric pressure, which means that there would be only one-third as much oxygen available. The greater pressure in the lungs would force a person to exhale forcefully about two-thirds of the air in their lungs. The oxygen content in the lungs would fall to one third of what it was due to the pressure change.
With so little oxygen now in the lungs, any oxygen in the blood that reaches the lungs is going to diffuse into the lungs instead of in the normal direction. Unconsciousness would quickly follow in just seconds instead of minutes. Getting to an oxygen mask quickly becomes a matter of life and death.
Now let's imagine what happens to a drowning person. They exhale air and inhale water. After just a couple of breaths, the lungs contain water instead of air. The passage of oxygen into the blood ceases immediately and unconsciousness occurs quickly.
If anyone has a better explanation I'd be delighted to hear it. I thought this through once because I was curious as to why one can't simply hold their breath during an airplane decompression event. The force of the air pressure in their lungs doesn't allow them to hold onto the life preserving oxygen.
The amount of air in their lungs is about a minutes worth. The instinct not to breath under water is so strong that it overcomes the agony of running out of air. No matter how desperate the drowning person is he doesn’t inhale until he’s on the verge of losing consciousness. At that point theres so much carbon dioxid in the blood, and so little oxygen that chemical sensors in the brain trigger an involuntary breath whether he’s underwater or not. That is called the “Breakpoint”; laboratory experiments have shown that the break point to come after 87 seconds. It’s sort of neurological optimism, as if the body were saying “Holding our breath is killing us and breathing in might not kill us, so we might as well breathe in. If the person hyperventilates first — as free divers do, and as a frantic person might — the break point comes as late as 140 seconds. Hyperventilation initially flushes carbon dioxide out of the system, so it takes that much longer to climb back up to critical levels.
Until the break point, a drowning person is said to be undergoing “voluntary apnea” choosing not to breathe. Lack of oxygen to the brain causes a sensation of darkness closing in from all sides as in a camera aperture stopping down. The panic of a drowning person is mixed with odd incredulity that is actually happening. Having never done it before, the body — and the mind– do not know how to die gracefully. The process is filled with desperation and awkwardness. So this is drowning a drowning person might think.. So this is how my life finally ends.
Along with the disbelief is an overwhelming sense of being wrenched from life at the most banal, inopportune moment imaginable… “I can’t die, I have tickets to next week’s gam” is not an impossible thought for someone who is drowning. These types of thoughts shriek through the mind during the minute or so that it takes a panicked person to run out of air. When the first involuntary breath occurs most people are still conscious, which is unfortunate, because the only thing more unpleasant than running out of air is breathing in water. At that point the person goes from voluntary to involuntary apnea and the drowning begins in earnest. A spasmodic breath drags water into he mouth and windpipe and then one of two things happen. In about 10 percent of people anything, anything, touching the vocal cords triggers an immediate contraction in the muscles around the larynx. In effect, the central nervous system judges something in the voicebox to be more of a threat than low oxygen levels in the blood and acts accordingly. This is called a laryngospasm. It’s so powerful that it overcomes the breathing reflex and eventually suffocates the person. A person with laryngospasm drowns without any water in his lungs.
In the other 90% of people, water floods the lungs and ends any waning transfer of oxygen to the blood. The clock is running down now. Half-conscious and enfeebled by oxygen depletion, the person is in no position to fight his way back up to the surface. THe very process of drowning makes it harder and harder not to drown, an exponential disaster curve similar to that of a sinking boat.
Occasionally someone makes it back from this dark world, though, and it’s from these people that we know what drowning feels like.