Image by hppyhntr via FlickrBreathing underwater is a skill generally reserved for little fishes. When we land mammals attempt it during scuba diving, we must proceed correctly.
Understand that oxygen converts to carbon dioxide only inside the lungs.
Know that everything else the throat, mouth and nose is dead-air space.
Realize that snorkels and scuba regulators increase the length of normal dead-air spaces.
Breathe deeply while diving, in order to flush out dead air and bring fresh oxygen into the lungs.
Inhale slowly (with your tongue at the roof of your mouth, like a splash guard) in case water has entered the snorkel or regulator.
Never hold your breath; trapped air will expand as you ascend.
If you begin to choke, hold the regulator with one hand and cough or spit into it as much as necessary; the regulator can handle it. Just don't panic.
Thanks to eHow