The cannons are the first to be used by a police force in Great Britain. Critics of Johnson say the purchase reeks of arrogance because Home Secretary Theresa May hasn't sanctioned the use of water cannons, according to the Guardian.
"The timing of this decision is driven by the enhanced risk that the water cannon, currently available to be purchased secondhand, are sold to another European police authority," reads a document on the mayor's policing and crime website. "The saving represented by this purchase is so significant as to justify the risk caused by the delay in the licensing process."
During an interview on LBC radio, Johnson reluctantly agreed to get blasted after initially saying, "I'm not quite sure whether I want to stand in front of a water cannon."
"OK. Man or mouse. Alright, you've challenged me to this. I suppose I'm going to have to do it now," Johnson said. "I'm doing this unfortunately from off-base, so I can see all my poor press people are going to be tearing their hair out. Never mind, it's got to be done."
Johnson purchased the cannons from the German federal police.