From BSA Health & Safety alerts:
As summertime swimming activities continue and youth become more comfortable in the water, it’s not uncommon for their water games to become more challenging and competitive. For qualified supervisors of Safe Swim Defense activities, for parents watching their children in the water, and for youth participants themselves, hyperventilation has been cited as a factor in numerous drowning incidents.
Breathing is triggered by a buildup of carbon dioxide rather than the depletion of oxygen. Holding your breath is an important stage in learning to swim, and the ability to swim underwater is an important skill for snorkeling and lifesaving. However, repeated deep and rapid breathing, or hyperventilation, before underwater swimming can result in a loss of consciousness by delaying the urge to breathe.
Contestants in underwater swimming events may be especially prone to this danger because, under the stress and excitement of competition, they may ignore their own built-in urge to breathe. In such cases, the swimmer may have little or no warning that he or she is about to pass out.
Caution anyone who is obviously breathing heavier than normal before they swim underwater. Because of concerns with hyperventilation, competitive underwater swimming events are not permitted in Scouting.