Dr. Peter G. Bourne, President Carter's controversial former chief adviser on health and drug abuse , has been hired by the United Nations to help plan a major U.N. program for development and conservation of worldwide water resources.

U.N. officials said yesterday that Bourne will work as a coordinating consultant for the U.N. Development Fund and the World Health Organization in setting up the program to be called the U.N. Decade for Water.

Bourne, a British-born psychiatrist and friend of the president, was forced to resign his White House post last July after giving an aide a prescription made out to a fictitious person for a tightly controlled drug.

Although the incident prompted a criminal investigation. No charges were preferred against Bourne or the assistant to whom he gave the prescription. Last December, Bourne was reprimanded publicly for his action by the state medically board in Georgia, where he holds his license to practice.

The U.N. officials said Bourne had been engaged to work on the water program under a two-year consulting contract. They added that his salary is still being negotiated and that his duties are still being defined.

However, the officials said, it is possible that he will become chairman of a committee coordinating the work on the water program of different U.N. agencies. Some officials said there also has been talk about Bourne's becoming chairman of a planned future special U.N. conference on water resources.

The U.N. officials said no special pressures had been brought by the Carter administration to obtain the post for Bourne. While conceding that he has no expertise in water conservation, they said Bourne was hired because he once headed the U.S. delegation to the U.N. Development Fund and is versed in dealing with the U.N. bureaucracy.

State Department officials said Bourne was one of several American citizens recommended to the development fund as qualified for the water job and various other positions. These officials also stressed that no special efforts were made by the department on Bourne's behalf.

The development fund, which will be Bourne's principal employer, is the U.N.'s largest technical assistance program for developing countries. In 1978, it had a budget of $415.8 million. The United States normally contributes 19 percent of its budget. CAPTION: Picture, PETER G. BOURNE . . . gets 2-year contract