On the face of it, it looks stupid. But it's not.
I was 13 when I was in a motel pool in Las Vegas. There was a kid who was "practicing holding his breath under water". It was his sister who finally realized that he had been down too long, and jumped in to pull him to the surface. She and I pulled him to the edge of the pool. No one there knew CPR. His parents were in some casino. I don't know if he lived or died.
From article "Water Hazards: Underwater Breath-Holding Can Be Fatal"
http://info.alaska.edu/risksafety/download/UE%20Publication/SD2002I3.doc :
"... prolonged underwater breath-holding is a very dangerous practice. It can lead to underwater blackout (also called shallow water blackout), inhalation of water, cardiac arrest, brain damage, and death. The danger stems from an insufficient amount of carbon dioxide to activate the body’s usual warning system.
"The impulse to breathe is triggered by the buildup of carbon dioxide in the blood, not by a lack of oxygen. If the level of carbon dioxide is not high enough, because of hyperventilation or some other reason, a swimmer or diver won’t get the message that it’s time to take a breath.
"Some swimmers and divers hyperventilate--take rapid and deep breaths—before going underwater in the mistaken belief that doing so will help them increase their lung capacity. Instead hyperventilation depletes their lungs of the carbon dioxide they need to tell them when to take another breath. Deep breathing as an underwater endurance step actually increases the risk of drowning.
"The U.S. Navy has issued several warnings about the risks of hyperventilating before going underwater. In one, following the drowning death of a chief petty officer in 1998, the service wrote, 'Hyperventilation is so dangerous that the U.S. Navy Diving Manual prohibits it and professional aquatics organizations caution against it.'
"... Can a lifeguard adequately supervise someone who is practicing extended underwater breath-holding? Submersion for even a half minute can lead to drowning, and longer than five minutes generally causes death. Monitoring would require continuous supervision, and monitoring underwater activities in a murky lake or ocean water is even more impractical. If a swimmer engaging in some risky activity, such as endurance breath-holding, asks a lifeguard to keep a special watch over him or her, the guard’s best response is to decline. The guard owes attention equally to all swimmers and should not agree to special monitoring of someone engaged in such a risky activity. Consider prohibiting swimmers from practicing endurance breath-holding in your facility. "
There's also an article, "Hypoxia, Carbon Monoxide Poisoning and Drowning" at
http://www.bbc.co.uk/dna/h2g2/A817850Sue