Sure, if I want to dive down into the underwater cave or explore a mineshaft with unbreathable air, I can announce that my character holds his or her breath. I would be fine for 1 + CON modifier minutes. And holding my breath is not an action, I can just do it.
But what if the transition from safe environment to hazardous environment happens all of a sudden? For example: a) I am drunk and fall off a ship; b) I get teleported to a plane with no breathable atmosphere; c) I get polymorphed into a beast that can only breathe underwater; etc.
In that case, the character wasn’t really holding his or her breath in that moment, because it all happened so fast. Does that mean he or she is out of air immediately and will fall to 0 hit points after a number of rounds equal to his or her CON modifier? But at any given point, you would have at least some oxygen in your lungs (and blood), even without holding your breath. Is this one of these question where the answer is “There is no rule, ask your GM?”. Because I am the GM and all these soliloquies are bothersome ^^.
If I spontaneously find myself in an environment I can’t breathe in, am I automatically holding my breath?