"Now, hold on," you're surely saying. "If it's just a contest to see who can hold their breath the longest, why not do it in a small tank of water, where they can easily sit up if they exceed their limits?" Oh, you naive fool. You're still not getting it: It's because freedivers are fucking crazy. Understand, the body changes in many ways when you go hundreds of feet deep with no air but what you hold in your body. In some competitions, half of the divers come up unconscious. A study of 57 freedivers in an eight-day competition saw a whopping 35 of them suffer from some "adverse event" or another due to the body freaking out because of the lack of air.
Which makes sense, as their body is all but completely failing on these dives. Like a robot running out of battery, the typical freediver's heart slows down to just 14 beats per minute, as opposed to the normal human heartbeat of 60 to 100 beats per minute. People in a coma have a faster beat. You shouldn't be able to maintain consciousness, let alone operate at that level. In fact, experts reportedly have little idea how 100 percent of the divers don't wind up unconscious on these dives.