You call the police reporting that you're being assaulted. They will show up. The assault will stop. Now, I agree they might just tell people off with no arrest but that really demands on mood, behaviour of people, location, and indeed damages to property.
Well, I am sorry that policing in your area is poor... But perhaps an actual crime in progress would elicit a different response.
I have called the police twice in recent years, once for a road traffic accident in my street and once for road rage and they showed up in under 5 minutes (UK). It's not possible to generalise, but I suspect that a report of an assault in progress should get a quick response (maybe don't mention the water guns too much ;) )
In most jurisdictions, it'd be both a) assault and b) something the cops wouldn't give a shit about unless it damages a laptop or something.