I thought a lot of this stuff was basically below conscious thought. You can interrupt it but otherwise the urge to breathe and the corresponding breathing proceeds apace. I've been told that in ventilator-dependent patients if the ventilator has too low a breath rate they still have the urge to breathe and feel it quite painfully until the ventilator takes a breath for them.
If you pass out under water because you run out of oxygen you will immediately resume breathing and drown.
