@Richman
having scubadived, I can say that at first time, you tends to breath fast then, as you calm down, your breathing slowly calm down until it becomes unconscious.
It's a weird experience, I'll be honest, the sens of weightlessness makes you feel you aren't properly in control of your body, until your brain adjust then it's exhilarating.
If you do it in rich sea floor you will be very impressed by the flora and fauna that skip past your vision, in gamer term it would be like being the first person to go through a massive graphical, gameplay, storyline upgrades, it's impressive, majestic, and you feel accomplished to be able to even witness it.