When you dive you breathe differently than you do on land.
TRUTH: Breathing naturally while underwater is one of the most terrific sensations you’ll ever experience, and one of the first things you’ll learn in your certification course. You will find that about the only difference between breathing air on land and underwater is that you must breathe through the regulator in your mouth – and since today’s regulators are so well made that breathing is made very simple and natural, even this part is easy.