Do they work there, though? I thought that France was famous for its regular large-scale protests, so it's not like water-cannons have killed off protesting.
The only solution in the long run to protests is to address their root cause. Anything else is a sticking plaster on a gunshot wound at best. Plus, water cannons are indiscriminate -- not everybody at a protest is an anarchist, not everybody deserves to be arrested or detained, but the water cannon can't tell the difference -- and sometimes even fatal, or cause serious injury.