The underwater breather utilizes electrolysis to extract oxygen from water. It is actually a simple idea, although I don't know of one that actually exists. There would be two terminals connected to an electric power source. When it is on, due to electrolysis, oxygen bubbles would come from one and hydrogen bubbles from the other. The oxygen bubbles would then be "harnessed" and you would breathe them.

This is only an idea that I have come up with; I don't know if it would work or not. Actually, because it doesn't exist today (that I know of), there's probably a good reason why it doesn't. I just thought I'd put it up here to see what everyone else thinks about it.