Holy smokes 20 meters just exhaling and no regulator..! Scary. Do our lungs can last that long ?
PS - you mentioned "no regulator" in you post above.
The regulator STAYS IN YOUR MOUTH until your reach the surface. Both during your OW class and in the unlikely event you ever need to actually do a CESA in the real world. Here's "the why" behind "the how."
If the Number 1 underwater rule in scuba diving is "NEVER hold your breath" the Number 2 rule is "ALWAYS keep your regulator in your mouth until you have something BETTER to replace it with."
If you are doing a CESA, it's because you're out of air. If you're out of air underwater, chances are there might be a strong desire to take a breath at some point before reaching the surface.
If you have a regulator in your mouth and try to inhale, you will get NOTHING as you try to breath against the vacuum of an empty tank. (Try it yourself on the pool deck during class; take a breath off a reg connected to a tank that is not turned on. Nothing.)
However, if you do not have a regulator in your mouth - and you try to take a breath - you will get two lungs full of water. The technical term for this is "drowning." You don't want to drown doing a CESA on a real dive. Your instructor REALLY doesn't want you to drown doing a CESA in class. (The amount of paperwork that creates is ridiculous...)
A second point to consider is that your tank is never
really empty. At depth your regulator is adjusting the pressure of the air coming out of your tank to match the ambient pressure at whatever depth you're at. Ultimately, your regulator can only deliver air if the pressure of the air inside your tank is HIGHER than the pressure of the water outside your tank. The deeper you are, the more pressure there is, the harder it is for the reg to deliver air. For instance, at 60ft/20m there is actually 44.7psi/3bar of air in your "empty" tank, but your reg can't get that air out of the tank since the pressure outside the tank at that depth is also 44.7psi/3bar. But as you ascend, the ambient pressure decreases. So as you approach the surface, the ambient pressure approaches 14.7psi/1bar. So if you left 60ft/20m with 44.7psi/3bar of air in your 'empty' tank - and you're now at 10ft/3m where the ambient pressure is roughly 20psi/1.33bar - your reg might actually be able to deliver a tad bit of air (10psi/0.66 bar) from your tank at ten feet that it couldn't deliver against the much higher pressure at 60ft. I wouldn't necessarily count on that air absolutely being available - because the example above doesn't factor in how the reg is adjusted, how efficient it's work-of-breathing is, etc - but the idea is that with the reg in, worse case scenario, you might get a pleasant surprise.
Keeping your reg in your mouth is true on not just the CESA, but every skill. OOA Drill? Don't remove your regulator until you have secured your buddy's regulator, have it properly oriented, and ready to put into your mouth. Hover with oral inflation? Don't remove your regulator until you have secured your low pressure inflator, have it properly oriented, and ready to put into your mouth. Switching to your backup regulator? Don't remove your primary regulator until you have secured your backup regulator, have it properly oriented, and ready to put into your mouth.
