They include providing clean water and sanitation to the poor, feeding a world population set to rise from seven billion to nine billion by 2050 and coping with the impact of global warming.
"Pressures on freshwater are rising, from the expanding needs of agriculture, food production and energy consumption to pollution and the weaknesses of water management," UN Secretary General Ban Ki-moon said in the report.
"Climate change is a real and growing threat. Without good planning and adaptation, hundreds of millions of people are at risk of hunger, disease, energy shortages and poverty."
The World Water Development Report is issued every three years to coincide with the World Water Forum.
Written by experts in hydrology, economics and social issues under the aegis of UNESCO, it aims to be the world's reference manual for water.
The document, the fourth in the series, made these points:
-- Population growth and a shift to more meat-intensive diet will drive up demand for food by some 70 per cent by 2050. Using current methods, this will lead to a nearly 20 per cent increase in global agricultural water consumption.
Farming today accounts for around 70 per cent of water use, ranging from 44 per cent in rich countries to more than 90 per cent in least developed economies.
-- Abstraction of aquifers has at least tripled in the past 50 years, supplying nearly half of all drinking water today. "In some hotspots, the availability of non-renewable groundwater resources has reached critical limits," says the report.
An aquifer is an underground layer of water-bearing rock or soil.
The report calls for an overhaul in water management and a massive effort to curb waste. Better irrigation systems, less thirsty crops and the use of "grey," meaning used, water to flush toilets are among the options.
-- The bill for coping with climate-induced water problems will be between 13.7 billion and 19.2 billion dollars annually between 2020 and 2050. This is based on the assumption UN climate talks limit global warming to two degrees Celsius.
"The current areas with water stress will be suffering more," said Olcay Unver, who coordinated the report, pointing as examples to the Middle East, South Asia and the south-western United States.
-- About 2.5 billion people have no access to decent sanitation, a figure meaning that a key Millennium Development Goal for 2015 is likely to be missed. In contrast, UN estimates last week said a goal for improving access to clean water would be met.