Hypoxic blackout
Hypoxic blackout—loss of consciousness under water—can affect anyone, even skilled, fit, competitive swimmers and free divers. Some individuals practice hypoxic training, to improve their breath-holding ability.

Hypoxic blackout—loss of consciousness under water—often affects skilled, fit, competitive swimmers and free divers (divers who don't use compressed air or scuba tanks).  These individuals commonly practice holding their breath, also known as hypoxic training, in water to improve their ability to hold their breath for increasingly longer periods of time.  But depending on how this technique is performed, it can be a dangerous practice that can result in brain damage or death.  Swimmers and free divers who hyperventilate before holding their breath for long periods underwater are at risk of hypoxic blackout. 

The urge to breathe is actually caused by a buildup of carbon dioxide (CO2) within your blood,  rather than the need for oxygen.  If you simply try to hold your breath underwater, the instinct to breathe will eventually take over, so there isn't a significant risk of "passing out" first.  The risk for hypoxic blackout increases when swimmers and free divers hyperventilate before submerging  underwater—either by rapid breathing or by taking deeper breaths.  Free divers know this method can enable them to hold their breath for longer periods of time.

It's the very act of hyperventilating that can be deadly.  When you hyperventilate before underwater swimming, you push more CO2 out of your lungs than with normal breathing, which diminishes the urge to breathe.  Once the oxygen in your bloodstream is used up, your brain stops functioning properly, and without warning you can lose consciousness.  At that point your system automatically forces you to take a breath, and water fills your lungs.  Unless rescue is immediate, brain damage and death are likely.

Training with instructors and with a skilled free-diving or swimming club will help reduce your risk of tragic accidents.  In addition, here are some other things you can do to avoid hypoxic blackouts:

• As emphasized above, don't hyperventilate before swimming under water
  

• Never swim alone
  

• Never "compete" to see who can hold their breath the longest
  

• When you're under water, don't ignore the urge to surface and breathe