If internet surfing is such a problem, why is productivity so high?
People need brain-breaks. It helps to get away from the desk, the normal routine, or whatever. That's why office naps really aren't a bad idea, even if they are only 15 minutes. Even if a person is on the computer all day an internet "break" is still just that, it's a change of brain use.
The more rejuvanation, the better the productivity. Now tell that to the people you work for. ;)