Why does it need a water canon?
Its to fight in a more humanitarian environment!
“The vessels of the class are armed with eight remote controlled guns of three different calibers (from 12.7mm to 127mm!) as well as with non-lethal weapons such as water cannons, sonic weapons and Light Emitting Diode (LED) Incapacitators for non provocative deterrence and defence.”
http://navalanalyses.blogspot.in/2016/05/baden-wurttemberg-class-frigates-of.html