Boris Johnson bought the three crowd-control weapons for £90,000 in 2014.
The following year then Home Secretary Theresa May refused to give permission for them to be used on safety grounds.
Current London mayor Sadiq Khan said £325,000 has been spent on them so far and money raised from their sale will help tackle gang crime.
The cost has been incurred by the Met's purchasing, fitting out and repairing the machines and includes £970 spent on installing radios and CD players.
Mr Khan hopes to save about £175,000 in maintenance costs across the remaining eight years of the equipment's lifespan.
The Ministry of Defence has been drafted in to help find a buyer.
Following the riots, which spread across cities in England after the police shooting of Mark Duggan in Tottenham, a report from the Inspectorate of Constabulary suggested water cannon and plastic bullets could be considered to deal with rioters throwing missiles and petrol bombs.
But Met Commissioner Bernard Hogan-Howe said: "We have seen them in Northern Ireland. They have been effective there, but they do have their limitations, they are not the answer."
In a statement to MPs in July 2015, Ms May said the cannon could cause serious injuries, including spinal fractures, and raised doubts over their usefulness in fast-moving riots.
Mr Khan said: "It beggars belief that such a huge amount of taxpayers' money has been wasted on paying to store these redundant machines."
Conservative London Assembly member Keith Prince said: "Although they cannot currently be routinely used, the Met can apply for a special licence to deploy water cannon were we to have a repeat of events like the 2011 riots.
"These water cannon have been used to train officers for deployment in Northern Ireland for the past two years and could provide a vital service on London's streets tomorrow if required."
Mr Johnson has not yet commented.
The controversial water cannon which former mayor of London Boris Johnson ordered following the 2011 riots are to be sold.