Which piece of equipment is crucial for monitoring your air supply while underwater?

The pressure gauge is essential for monitoring your air supply while underwater because it directly shows the remaining air in your tank. Understanding how much air you have left is critical for planning your dives, managing your ascent, and ensuring that you surface safely without running out of air.

While the depth gauge indicates how deep you are—important for safety and for understanding your dive profile—and the compass helps with navigation, these tools do not provide information about your air supply. Similarly, the buoyancy control device aids in managing your buoyancy but does not indicate the amount of air remaining in your tank. Therefore, the pressure gauge's role in monitoring your air supply is vital for the safety and effectiveness of your dive.