What is the most important rule in scuba diving?

1.plan your dive

2 always have a buddy

Breathe continuously, and never hold your breath!

Regularly check the air supply

Never hold your breath. Keep breathing at all times!

1. Plan your dive

2. Do Not dive alone

first is your health and fitness level,need to be strong as you well  have to carry the heavy air and heavy cylinder and scuba gears which might weigh u down,also relax ur body your self.

you must dive two or morehave a signal buoy and a dagger

Just relax. No skip breathing. No rapid ascend.