Nothing important. It seems I'm the only one who think this weird. So free apnea divers will continue to emit breathes under water!

No offense, but I'm guessing English isn't your first language. I am assuming you're saying that the game sounds like you are scuba diving even though you don't have the gear? inhaling and exhaling?
I have to disagree here. I just tested it with NO mods. full scuba gear sounds drastically different from non at all. With scuba gear you can hear loud breaths. Without any scuba gear, you only hear water motion.

Id say its very minor and it doesn't correlate with what you said in the other ticket, about divers have CO2 in their tanks?

That's exactly what i'd like to mention.
I agree with you, there is a difference between scuba or free diving sounds.
But i disagree when you write "without any scuba gear, you only hear water motion".
If your player moves or stays quiet into water, there is always the same bubbles sounds at a breath frequency, non-correlated with arms movement. If you free dive a little bit, you'll notice you never have permanent, regular bubbles sounds like these.
But as you said, it's very minor hint. You can close it.
Sorry for my poor English and my pathetic try to make a joke comparing Arma' free diving to scuba diving with a CO2 tank.