While it might seem like magic, breathing underwater is one of the easiest things you will do. Breathe slowly and continuously, and never hold your breath. When you breathe in, your regulator delivers as much air as you need. When you breathe out, the air just passes through the regulator and into the water.
You will be trained to remove, recover, and replace your regulator to ensure that you are prepared in the event your regulator comes out of your mouth.