I think I am going to give my date some tap water to drink.
Is there a specific reason that you are going to give them tap water?
Well I simply forgot all of my bottled water back at home.
If you have nothing better to offer, then it's fine to give your date tap water. However, you should be honest with her about what happened, so that she won't think you are being inconsiderate.
This is fucking awesome.
Honesty is normally the best policy. Telling your date about the tap water issue is just the best way to go about things!