This is what I was hoping for. Good general discussion. I had also thought of the one way valve idea, just hadn't mentioned it yet. As far as water pressure and being able to breathe, isn't that why we have a rib cage? I would think the ribs would maintain room for some volume of air at minimal depths?
Med, you are right, I was only talking about being under a couple feet just to be able to inspect and maybe clean without having to surface at regular intervals.