Africa needs water for farming as well as for satisfying people's own needs
Millions of the world's poorest people survive on less than a fifth of the water they need, the United Nations says.
The UN says guaranteeing proper water supplies is vital to eradicating poverty.
It believes the problem is worst in Africa, which also needs huge quantities of water for agriculture. While the world population tripled in the last century, the use of renewable water resources grew six-fold.
The warning comes in a report written by a Dutch adviser to UN Secretary-General Kofi Annan.
The adviser, the Prince of Orange, prepared the report, No Water, No Future, for a conference being held in the Ghanaian capital, Accra, from 15 to 17 April.
Just getting by
He says: "The world is in a water crisis, with the problem perhaps most acute in Africa.
"Globally, more than one billion people do not have access to safe drinking water, and half of the world's population lacks adequate sanitation. More than two million people die annually from water-related diseases."
The UN says the absolute daily minimum amount of water a person needs is 50 litres (13.2 gallons) - five litres for drinking, 20 for sanitation and hygiene, 15 for bathing and 10 for preparing food.
Millions of people try to exist on 10 litres a day (2.6 gallons). They live in Gambia, Haiti, Djibouti, Somalia, Mali, Cambodia, Mozambique, Uganda, Tanzania, Ethiopia, Eritrea, Albania and Bhutan.
In 27 more countries, people try to manage on a daily average of 30 litres (eight gallons) or less.
Death rates slashed
The Prince says people need water to grow food as well as for their own use. Agriculture accounts for about 35% of Africa's gross national product, 40% of its exports, and 70% of its employment.
He says: "No single type of intervention has had greater overall impact upon economic development and public health than the provision of safe drinking water and proper sanitation.
"They can reduce the morbidity and mortality rates of some of the most serious water-related diseases by up to 80%".
The Prince says the world must reach three targets:
Known as "a Marshall Plan for Africa", it aims to commit the continent's leaders to working for development, establishing good governance and avoiding conflict.
Nepad's supporters say a programme to promote co-operation among 10 nations in the Nile basin has been producing good results, and could serve as an example to the rest of Africa.
Yet more than 200 million Africans are chronically undernourished, life expectancy is only 54 years, and malaria is now killing 900,000 people annually across the continent.