Farming poses the biggest threat to fresh water supplies, according to a major United Nations report.
Failure to protect water will worsen poverty, the report warns
Agriculture was consuming more water as the world population increased and as people turned to a Western diet, one of the scientists on the report said.
Farms use two-thirds of fresh water taken from aquifers and other sources.
The UN concludes that ending subsidies on pesticides and fertilisers, and realistic pricing on water, would reduce demand and pollution.
A lack of adequate protective measures now will lead to greater problems in the future, warns the report, entitled Challenges to International Waters: Regional Assessments in a Global Perspective.
Co-ordinated by the United Nations Environment Programme (Unep), it brings together evidence from about 1,500 researchers throughout the world.
It was released during the World Water Forum, held this year in Mexico.
Failure to protect fresh water reserves will lead to reductions in river flows, increased salinity of estuaries, and loss of freshwater plants and animals including some species used as human food, it says.
Farming and fishing
There is mixed news on fisheries, with protection measures working in some areas of the world but concern over destructive fishing methods such as explosives used in other regions, notably East Asia.
Fishermen can generate a 200-fold instant return on investment using explosives, but the area fished is then rendered useless.
Unep also points to subsidies as a factor in degradation of fresh water resources and the marine ecosystem.
Destructive fishing and nutrient runoff threaten reefs, the UN says
Subsidies for fleets can promote otherwise uneconomical fishing, while on farms they can encourage careless use of pesticides and fertilisers, which can in turn run into the sea and damage coral reefs.
"There are many important messages emerging from this pioneering study," said Unep's executive director Klaus Toepfer.
"One that rings loud and clear is the economic one - that our collective failure to value the goods and services provided by international waters, and to narrowly price the benefits in terms of the few rather than the many, is impoverishing us all."
The dilemma facing developing countries is made clear by the contradiction between this report and another released earlier in the World Water Forum by a different UN agency, the African Development Bank (ADB).
The ADB urges greater exploitation of water resources - more irrigation for farms, more dams for hydropower - in order to promote Africa's economic and social development.
Unep, by contrast, is highlighting the long-term environmental damage which would result if development is unsustainable, and pointing up the economic costs which would eventually ensue.