Why can't humans live (breathe!) under water?

We have been thinking about what changes (adaptations) might allow humans to live in an under water environment ... We think we'd do pretty well as mermaids and men if only we didn't need air - in particular, oxygen - (and so, scuba tanks)!

This is because people breathe oxygen gas as O2 which is in a different form from the oxygen molecules (tiny tiny parts!) found in H2O (water), and easier for us to absorb than the dissolved O2 gas bubbles (which fish can "breathe" through their gills). If you'd like to look into the science more closely this is a handy link ...

http://science.howstuffworks.com/question386.htm

Of course we can get around this by using a whole lot of artificial (made by us!) technologies. Do you recognise these ones?

 Or could we just swallow gilly weed like Harry Potter and grow gills and webbed hands and feet ... ? What do you think :)?