A few things, when I was taught to swim, we were taught that while our heads were under water we should open our mouths to blow out air while swimming, and when you run out of air it's time to bring your head back out of the water to get more air. So you can open your mouth under water. Also we've established in the narutoverse that the laws of this world are not applicable. Especially as seen in the Zabuza arc.
Two I think Water Encampment Pillar mixed with the amount of lava she used against Madara initially should due a rather good job of providing protection.
Not quite sure what you're trying to imply. I am aware you can open your mouth under water, i mean duhh. My point was, how is she going to get the mist to release out of her mouth when water will be rushing in. If what you meant was since you can blow carbon dioxide out your mouth/nose she can do the same with her mist, no. What happens when you blow into mist? Will the mist stay still or blow away? Same with the water pushing it back into her mouth. Also were talking about Water Dome, she isn't going to be, at any time, sticking her head out of the water for air.
@ Bold - You're right, but we haven't seen this actually happen, so we use logic, until proven otherwise by the manga.
