Visiting Prague on a trip round Europe I vividly remember being warned that if our party were too noisy outside a residents window at night we might have eggs dropped on us. We certainly didn't make that mistake.
Now I live in a flat in a UK city, in a student area. It is not an uncommon occurrence for someone to stop and have a conversation outside my window in the wee hours. I have even been treated to drunken guitar renditions of pop songs, that where not that great to begin with. It's because there is a sheltered archway there, and so its a nice dry place to stop on the way home from a good night out. Most of the time, if asked to move on, people will oblige. If they are particularly drunk, however, they may decide that they would rather stay right outside my window. Furthermore, being drunk doesn't noticeably make them any quieter.
I could call the police (non-emergancy) at this point, this behaviour probably counts as "drunk and disorderly". But at 2 in the morning, I really just want to get back to sleep. The police probably have better things to do with their time as well.
So would there be legal ramifications to employing a water pistol in this situation?