"It will happen," Captain Cousteau was saying. "Surgery will affix a set of artificial gills to man's circulatory system-- right here at the neck-- which will permit him to breathe oxygen from the water like a fish. Then the lungs will be by-passed and he will be able to live and breathe in any depth for any amount of time without harm.

"Do you realize what that will mean? He will be able to observe, train, cultivate, and exploit the seas at first-hand. Maybe the first man will be an undersea farmer, or miner, or rancher. Maybe just a scientist. At any rate, there will be no depth-time barrier, we know that. When his duties are done, he will be rehabilitated to air breathing by more surgery. It will happen, I promise you."

Captain Jacques-Yves Cousteau

on Homo Aquaticus

Cousteau pretty much hit the nail on the head when he talked about gills some sixty years ago.  You're not going to get very far under the sea if you insist on such old-fashioned tastes as breathing air.   You have to move with the times and start sucking liquid down your lungs.

But scientists are an observant lot and they noticed pretty quickly that when people tried breathing water they ended up very dead.  So, in the 1960s they tried a different liquid: oxygenated fluorocarbons.  These made for really cool demonstrations where you could stick a mouse in beaker of liquid and it would stay alive-- cheesed off, but alive.   On closer examination, however, the fluorocarbons weren't as neat a solution as imagined.  First, you have to oxygenate the liquid anyway, so the problem just gets shifted down a notch.  And second, trying to breathe even an oxygenated liquid, isn't easy.  The human lungs aren't designed to pump liquid and the fluorocarbons didn't need to just bring in oxygen, but remove CO², which they were rather poor at. 

Also, trying to get prospective volunteers to suck in a lungful of liquid resulted in what is known as "consumer resistance."

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