The most important rule in SCUBA is never hold your breath.
In the classroom and pool sessions that I teach, I always tell the students that by the end of time we have together they will have heard me talk about the most important rule in SCUBA more than twenty times. I do this so that they will focus on what the words mean. By the end of the second pool session they recite the answer to the question loud and proud.
During the past weekend, I had to work with students who were trained by other instructors. When I asked what the most important rule in SCUBA is, some answered that "you should always breathe".
The problem was that at the time we were talking about regulator recoveries. Of course I made light of that answer since it is very inadvisable to breathe underwater while the regulator is out of your mouth (just so you know, I meant this as humor).
Some people might feel that there is a distinction without a difference between “never hold your breath” and “always breathe”. In other words, some people think that the two terms mean the same thing. I do not agree.
Always breathe implies that you will always have a source of air available. However never hold your breath indicates to the students that they should always keep their airway open. The concern is that a diver could suffer lung overexpansion injuries when holding their breath upon ascent.