See, once they can no longer hold their initial breath and start to hyperventilate, they might aspirate water, triggering something called a laryngospasm.
That's perfect, Captain Nemo, but the air you carry must be quickly depleted; and once it contains no more than 15% oxygen, it becomes unfit for breathing.
And one of the key things before you go in to do the settling is you have to stand outside and take like three really deep breaths before entering the room.