Mr Johnson denied "playing politics" by purchasing the riot control equipment before its use on the streets of mainland Britain has been approved by Home Secretary Theresa May.
Mr Johnson and Mrs May have been identified in polls as leading rivals to succeed David Cameron as Conservative leader, but the Mayor insisted that his decision was not an act of defiance to the Home Secretary at a time when she has been embroiled in a public row over extremism in schools.
He told LBC 97.3 radio that by spending £130,000 on buying, refitting and transporting three second-hand vehicles from German police now, before they are approved for use, he will save Londoners £2.4 million in the long term.
Water cannon have never been used on the British mainland, although they have been deployed in Northern Ireland.
But the Metropolitan Police concluded there could be a limited role for the tactic in the wake of the 2011 riots, and Mr Johnson believes there is broad support from Londoners despite strong opposition in some quarters.
Mrs May insisted that she would carry out a "proper" consideration of the health and safety implications of water cannon before reaching a decision on the police request for permission to use them in England and Wales.
But a spokeswoman for the Mayor's Office for Policing and Crime (MPAC) said the German authorities had threatened to sell them elsewhere if the deal was not completed by July.
Challenged by interviewer Nick Ferrari to be filmed standing in front of the water cannon, Mr Johnson initially seemed reluctant, telling him: "I'm certainly prepared to do anything to show that they're safe within reason. I'm not quite sure whether I want to stand in front of a water cannon, I haven't done anything to deserve it."
But after a few moments' consideration, he came round to the idea, telling Ferrari: "OK. Man or mouse. Alright, you've challenged me to this. I suppose I'm going to have to do it now.
"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. Thanks for that one, Ferrari!"
Mr Ferrari suggested that he and the Mayor could stand in front of the cannon together, wearing bright yellow fishermen's oilskin and sou'wester to protect them from the water.
Asked whether she would clear the cannon for use, Mrs May said: "There is a decision to be taken about water cannon and I will make sure that that decision is taken on the right basis. There are health and safety issues that need to be looked at very carefully.
"But let me clear about the timetable. The Metropolitan Police started talking about water cannon after the riots in August 2011. The police have to put a case for water cannon. They didn't put a group together to do that until January 2013 and they did not submit that case to the Home Office until March 2014.
"I will be taking a decision and I will be taking it on the right basis. There are difficult issues here - health and safety issues - that need to be considered properly."
An independent ethics panel, headed by barrister Lord Carlile, will look at when water cannon should be used if they are licensed by Mrs May - who has already ruled out providing central government funding.
Labour and the Liberal Democrats have condemned Mr Johnson's decision as "reckless", insisting there is not enough evidence that the tactic is effective in maintaining order.
Challenged on LBC over whether he was playing politics by generating a "dust up" with his putative leadership rival, Mr Johnson said: "No, no, no, no. What we're doing is we're trying to save some money.
"From my point of view as Mayor, the question is really could we legitimately deprive the police of this tool of crowd management when that they say they might need it.
"If you consider that there could be situations in which people's property was very badly damaged or even lives were put at risk as a result of them not having that equipment, it seemed to me that on balance that it was right to go with the police. We had a consultation - I think Londoners were pretty overwhelmingly in support of the measure."
He acknowledged the Metropolitan Police would not be able to deploy the equipment until Mrs May has given her approval, but added: "I think it highly likely that approval will be granted. The problem was that if we waited and we were continually waiting for this approval, if we waited any longer, we would've missed the window to buy them for the very good price that we've got. So we've saved Londoners £2.4 million."