What happens if I use up all my air?

That’s not likely because you have a gauge that tells you how much air you have at all times. This way, you can return to the surface with a safety reserve remaining.

But to answer the question, if you run out of air, your buddy has a spare mouthpiece that allows you to share a single air supply while swimming to the surface. There are also other options you’ll learn in your scuba diving training courses in Perth.