Humans cannot breathe underwater because our lungs do not have enough surface area to absorb enough oxygen from water, and the lining in our lungs is adapted to handle air rather than water. However, there have been experiments with humans breathing other liquids, like fluorocarbons.
How do you actually breathe underwater?
Breathe Out - Most novice swimmers tend to hold their breath underwater instead of breathing out when swimming. When your face is submerged in water, you should be breathing out gently and bubbles should come out of your mouth or nose. Breathe In - Most swimmers breath in through their mouth.
Can humans breathe pure oxygen?
Oxygen radicals harm the fats, protein and DNA in your body. This damages your eyes so you cant see properly, and your lungs, so you cant breathe normally. So breathing pure oxygen is quite dangerous. Astronauts and deep-sea scuba divers sometimes breathe pure oxygen because they work in very dangerous places.