Why can't humans breathe underwater?

When we go swimming, we hold our breath under the water. It is just a habit that we all do but why can't we breathe? Water has oxygen in it as it is H2O so surely it could go round our bodies like air. Also, water is just another state of air and the molecules are a bit further apart. But apparently this makes a big difference on what we can and can't breathe.

Water is made of hydrogen and oxygen gas. The reason that we can't breathe water is is because the oxygen is bonded together with 2 atoms of hydrogen and the oxygen is then useless to our lungs in this form. Also, humans can't breathe underwater because our lungs don't have enough surface area to absorb enough oxygen from water as there is not that much in it. The lining of our lungs, also is adapted to handle air rather than water. However, scientists have done experiments where they make humans breathe other liquids like fluorocarbons. Fluorocarbons can dissolve enough oxygen so that we can breathe.

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