A London Assembly report has found that the Metropolitan Police has failed to make a "convincing case" for buying three water cannon.
The police and crime committee says that, with no specific intelligence of disorder, it is unclear why there is a "rush" to get them.
London Mayor Boris Johnson is consulting on plans to pay up to £100,000 each for three second-hand water cannon from Germany.
The Met has said the water cannon would be "rarely seen and rarely used" and it claims to have public support for the idea.
The vehicles have been used in Northern Ireland but they are not currently authorised in mainland Britain.
The home secretary must approve their introduction in England and Wales.
The Environment Committee will quiz London Assembly experts and the Environment Agency today about whether the capital is sufficiently equipped to deal with future rising water levels and flood threats.
Experts from the Environment Agency, the National Flood Florum and the London Assembly will answer questions on the sustainability of London's flood defences and the capital's readiness to react in an emergency.
Mayor Boris Johnson's controversial plans to allow the Metropolitan Police to purchase water cannon will come under scrutiny at City Hall for a second day.
Assistant Commissioner Mark Rowley and Deputy Mayor for Policing and Crime Stephen Greenhalgh will appear before the Assembly today to answer questions on the Mayor's proposals.
Chair of the Police and Crime Committee Joanne McCartney AM said: "Met Commissioner Sir Bernard Hogan-Howe has said water cannon in London would be 'rarely used and rarely seen'. We have yet to hear a convincing case for when and how they would improve safety for all on London's streets."
The London Assembly budget committee will discuss aspects of the Mayor's draft budget for 2014/15 today.
Labour has criticised the Mayor for failing to address London's "cost of living crisis", claiming that his proposed cut of the Mayoral council tax precept is meaningless compared to the impending rise in travel tickets.
London's economy is missing out on skills because parents - particularly women - are poorly served by the capital's labour market, which often forces them into lower-paid, less secure part-time jobs, the London Assembly has claimed today.
A new paper from the Assembly's Economy Committee says a lack of affordable flexible childcare is also hindering parents from working in the capital. The Committee claims this represents a waste of a significant proportion of London's skilled workforce.
The head of Scotland Yard admitted his force has been damaged by the Plebgate controversy but defended his own handling of the affair.
Metropolitan Police Commissioner Sir Bernard Hogan Howe said there was "no doubt some damage had been done".
He also insisted crime statistics were "generally sound" despite investigations into serious allegations that officers are manipulating them to improve performance records.
One officer is being prosecuted and eight face disciplinary action in the wake of the row over claims - which he disputes - that the then cabinet minister Andrew Mitchell called officers "plebs".
Sir Bernard said soon after the incident that his officers "accurately reported what had happened".
The London Assembly Police and Crime Committee will question the Metropolitan Police Commissioner Sir Bernard Hogan-Howe on the work of the Metropolitan police later today.
Deputy Commissioner Craig Mackey and Deputy Mayor for Policing and Crime Stephen Greenhalgh will also appear.
Crime statistics and student protests are just two of the subjects to be discussed at the meeting.
Regeneration activities planned for the Olympic Park site will only be able to go ahead if millions of pounds of additional funding is set aside, according to a report published today.
The report from the London Assembly's Regeneration Committee warns that community events could be cut if extra funding is not secured.
It argues that the Mayor should set aside at least £12m more funding for the London Legacy Development Corporation between 2015 and 2017 to ensure the planned activities go ahead.
A report published today from London Assembly Green Party member Jenny Jones outlines key reforms that could make London's roads safer for pedestrians and cyclists.
The proposals call for authorities to assume injured pedestrians or cyclists deserve compensation unless it can be proved otherwise.
It also calls for the laws to be adjusted to encourage greater use of driving bans for longer durations.
Extra moorings are needed on the capital's waterways to deal with an influx of new boat owners.
That's according to a London Assembly report which says more needs to be done to help stop overcrowding as more cash-strapped Londoners turn to a life on the water.