One drill that I did as part of a tech instructor course was to lay a 50ft line down at 170ft, then take off all my gear except suit and weightbelt and leave it on the bottom (so no mask either), swim to the other end of the line to where someone else was waiting, locate a suitable regulator (no mask remember), make sure the gas was on or turn it on, and take no more than five complete breaths, then swim back up the line to my own gear, which had been rearranged and separated whilst I was away and regs turned off. Identify the reg I wanted to breathe from and turn on that reg first. Then don the gear (twinset plus two stages).
In case this sounds like lunacy there were two people at each end and my instructor trainer was closely shadowing me the entire time. At any point I could have asked for breathing gas and would have been given it, but I would have failed the exercise. At each end the sole role of one of the two people was to make sure I didn't pick a regulator supplying gas that would be unbreathable at that depth. So it was actually perfectly safe, but none the less stressful.