Many summers ago, myself, Mark and a couple of our mates used to go out in his car, armed with water guns and shoot basically anyone we came across. Depending on who you shoot and how hot it is, you'd be surprised how many people will cheer.

To answer your question though, I think it probably is assault, but I doubt anyone would get very far in terms of actual prosection. I suggest waiting until you're fast enough to run away ;)