water cannon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A truck-mounted apparatus that fires water at high pressure, used especially to disperse crowds or control rioters.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device that shoots a large amount water at high pressure through a hose-like tube.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a hose (carried on a truck) that fires water under high pressure to disperse crowds (especially crowds of rioters)