Sometimes I mention scuba diving, and people react with an obvious aversion to even attempting to breathe underwater. I remember needing an extra second or two of courage, but I trusted my equipment.
And, here’s why:
First of all, your instructor does everything first. You observe underwater breathing up close and personal BEFORE you attempt it yourself.
Second, you take your first breaths above water. Your feet are firmly on the bottom of the pool, your head is above water, and you inhale/exhale through your regulator. It works!
Finally, with your feet still firmly on the bottom of the pool, you bend at the knees and lower your head below the water surface.
At this point, you know that if something goes wrong you can simply stand up. But, nothing goes wrong. The air you were breathing through your regulator just a moment ago is still available to you.
Keep in mind that the regulator is inside your mouth. Your mouth is keeping the water out. And, through the miracle of a simple valve, air goes one way into your lungs, and then out into the water as lots and lots of bubbles.
So, the key to starting scuba diving is a short series of baby steps. You watch someone else do it first, then you breathe above water, and then, after confirming that you do, in fact, have an air supply, then and only then you submerge.
Then you surface and start shouting to your instructor about how awesome it is to breathe underwater!