water cannon 
noun
A device that ejects a powerful jet of water, typically used to disperse a crowd.Hundreds of rioters launched a sustained assault along the West Circular Road before a water cannon and plastic bullets were used to disperse the mob....
- After the demonstrators threw rocks and paint bombs, police responded with overwhelming force, firing rubber bullets, water cannon and tear gas before launching baton charges.
- A team of police officers from the Central Jakarta Police precinct cordoned off the area with three buses and a truck to stop the brawl, and used a water cannon to disperse the mob.