Eat Chat (not chatter that's Bengali) and don't worry about the water (its Bisleri anyways).
When in Delhi, chat is a must. Before you wince, let me tell you that Delhi Belly is an urban legend. Chat does not screw up your belly. Its the loosies that you have to deal with the morning after.
There are generally two types of chat food. One is gol guppa. That's a category in itself. You lean forward and eat those small things dripping with strange liquids. And all this has to be eaten standing in a puddle of slosh created by earlier customers. The trick is to look heaven wards and focus on the strange sensations in your mouth.
The other stuff is what they call chat. I know this is really confusing but the experience is not. They taste the same. Except that chat has curd thrown to soften the blow on your stomach.
And then there are other stuff like kulcha channa. Those are not really chat but you can eat that too anyway.
Recently other street food like momos and bhel puri have caught the fancy of the city. But only a chat can leave a bad taste in your mouth and give you a bad morning after.
Last but not the least - as you finish your chat throw the plate (if plastic or paper) onto the ground and let out a loud sound of satisfaction. This completes your second step to being a dilliwali...