Rising demand for energy is posing a threat to freshwater supplies already under strain from climate change.
It urged energy companies to do more to limit use of water in everything from cooling coal-fired power plants, to fracking for shale gas, to irrigation for crops grown to produce biofuels.
By 2030, the world will need 40 per cent more water and 50 per cent more energy than now, the report said, but water is under pressure from factors such as a rising population, pollution and droughts, floods and heatwaves linked to global warming.
"Demand for energy and freshwater will increase significantly in the coming decades," UN agencies said in the World Water Development Report. "This increase will present big challenges and strain resources in nearly all regions."
Around the world, about 770 million of the world's 7 billion people now lack access to safe drinking water, it said, and the energy sector accounts for about 15 per cent of water withdrawals from sources such as rivers, lakes and aquifers.
"This interdependence calls for vastly improved cooperation" between water and energy, said Irina Bokova, Director-General of UNESCO (UN Educational, Scientific and Cultural Organization).
The report, launched the day before the UN’s World Water Day, lamented the lack of influence of the water sector compared to what it called the "great political clout" of energy – the production of which uses water across the spectrum of technologies, often as a coolant.
Least water was used in wind and solar power, while heavy users included hydraulic fracking to produce shale gas or the extraction of oil from tar sands. Hydropower dams were sometimes built with little thought for other water users and the report urged caution about biofuels, partly because of water used for irrigation.
"China and India, the world's two largest producers and consumers of many agricultural commodities, already face severe water limitations in agricultural production, yet both have initiated programmes to boost biofuel production," it said.
Zafar Adeel, head of the UN University's Institute for Water, Environment and Health, said that governments should re-think subsidies for both energy and water.
"Pricing water is much more challenging" than energy, he told a telephone news briefing. The UN General Assembly declared water a human right in 2010, strengthening arguments that basic supplies should be free.
Energy companies say they try to limit water use. Exxon Mobil, for instance, said that net freshwater consumption at its operations fell 11 per cent to 2.1 billion barrels in 2012 from 2011.
The UN study said there were examples where energy could successfully recycle water. In Stockholm, buses and taxes run on biogas produced from waste water, which is rich in methane.
A draft report by the UN's Intergovernmental Panel on Climate Change, due for release on March 29, says that global warming will disrupt water supplies, especially in developing nations, with damaging impacts from food to health.