Why can't we breathe underwater?

Wow, I did not realize it has been a month since I last posted.

Zack asked a question today which led to some Internet searching which led to more questions and so on. His original question was "If we drink water why can't we breathe underwater?" Seriously? He is six, where are these types of questions coming from? I, being the not so scientific type HAD to look that up. When trying to explain the info that I found he started asking more questions like why does our bodies need the water?... what happens if we don't get enough water?...will humans evolve into super humans?...I can not wait for school to start back in session so he can ask his teacher these kinds of questions and I don't have to feel so much like an idiot not knowing the answers.