Hold breath or breathe out?

If you empty the lung completely before breathing in air (as often suggested), you are assuming you will be able to breathe in air the next moment when you have to breathe in; i.e., you would rely on the (hopeful) fact that you will be able to get your mouth out of water the next moment. What if something not in your control happens and you can't get your mouth out of water soon enough (e.g. for some newbie who can't act soon enough, or whatever...) You will be "breathing" in water, no? Because of this concern, I always keep a little "reserve" of air in my lung before breathing in. Is this a justified concern? You really have to push hard to get all of the air out (as anyone who uses an inhaler knows). I don't think that most people who breathe out while their face is in the water do that. I certainly don't. But it is true, as you say, that you can't always rely on getting a breath when you want one, depending on how wavy the water is.

If you empty the lung completely before breathing in air (as often suggested), you are assuming you will be able to breathe in air the next moment when you have to breathe in; i.e., you would rely on the (hopeful) fact that you will be able to get your mouth out of water the next moment. What if something not in your control happens and you can't get your mouth out of water soon enough (e.g. for some newbie who can't act soon enough, or whatever...) You will be "breathing" in water, no? Because of this concern, I always keep a little "reserve" of air in my lung before breathing in. Is this a justified concern? You really have to push hard to get all of the air out (as anyone who uses an inhaler knows). I don't think that most people who breathe out while their face is in the water do that. I certainly don't. But it is true, as you say, that you can't always rely on getting a breath when you want one, depending on how wavy the water is.