S.C.U.B.A. means Self Contained Underwater Breathing Aparatus.
Which means you can breath underwater. That is exactly what all the exciting is all about and makes it all possible. We as humans are not designed to live underwater but thanks to some genius people, we can.
Because it is adventurous and fun, you need to start with behaving underwater. So you need to know how your equipment works and how to use it. Then you need to know some things about the physical laws underwater, the effect on our body. And then you need to know something about the place you go into the water.
It is a lot of fun, once you start learning, a completely new world will open.
To start you have some options...