From the time they are children squirming in diner benches with their parents, New Yorkers are told to drink their water because it is the best anywhere. A report issued today by a number of environmental organizations that make up the Clean Drinking Water Coalition pretty much confirms that impression — at least in terms of purity, cleanliness and safety.

“Over all, we’re pleased to report that New York City drinking water continues to be of very high quality,” said James L. Simpson, an author of the report and a staff lawyer with Riverkeeper, an environmental advocacy group devoted to the Hudson River. “Over all, the city gets good grades in protecting our water supplies but we also found areas that need improvement.”

The report (with no mention, specifically, of the politics of tap water) is in the form of a grade school report card. And on most areas — including controlling waterborne disease and keeping ducks and geese from fouling the waters — the city was given A’s or B’s.

But there were a number of C grades, too. Most significantly, the city got a C-minus in two critical areas: land purchases around the heavily developed Croton watershed, mostly in Westchester County, and a program to upgrade more than 100 local sewage treatment plants. (The department says that 40 of those plants are in the Catskill Mountain watershed and that almost all of them have been upgraded.)

Mr. Simpson said that the city simply has not bought enough land around the Croton system to protect the water there from runoff from driveways and lawns. The Croton system accounts for only 10 percent of the 1.1 billion gallons of water used every day by the city, and in terms of quality, it is the least acceptable water in the whole system. The city is spending $2.8 billion to build a gigantic filtration plant in the Bronx to filter water from the Croton system. The remaining 90 percent of the city’s water comes from the Catskill mountains and is not filtered.

But the substandard sewage treatment plants belonging to small municipalities in the Catskill Mountains could threaten the purity of the city’s water. Mr. Simpson said the city is supposed to be providing money and technical support to upgrade those systems but has been falling behind on that schedule.

Mr. Simpson said the most troubling finding in the report was the continuing problems that the city is having controlling turbidity in Catskill reservoirs. Turbidity refers to cloudiness in the water caused by sand and mud that washes into the water after storms. Turbidity in and of itself does not pose a health hazard — although aesthetically it’s a turnoff. But it can interfere with disinfection because parasites can hid behind the sediment.

The federal Environmental Protection Agency has warned the city that unless the turbidity issue is controlled, New York is at risk of losing a federally granted exemption from clean water laws requiring drinking water supplies to be filtered. New York is one of only five cities in the country to be exempted from this rule.

Last year, New York’s exemption was extended for 10 years, on the condition that drinking water continue to meet very high standards. If the city were to lose that exemption, it would be forced to build a filtration plant for the remaining 90 percent of its water at a cost estimated to be greater than $8 billion.

Besides Riverkeeper, the other groups in the drinking water coalition that prepared the report are the New York Public Interest Research Group and the Catskill Center for Conservation and Development.

The groups have no authority over the city’s Department of Environmental Protection, which runs the water system. But Mr. Simpson said that he hoped the city would heed its findings. And the group intends to issue a similar report every years.

Mercedes Padilla, a spokeswoman for the department, said in a statement:

D.E.P. is gratified that the Clean Drinking Water Coalition has recognized the accomplishments of the City’s active watershed protection program. New York City’s water is among the best in the world and to keep it that way, the City to date has spent or committed nearly $1.5 billion to watershed protection, which includes acquiring more than 85,000 acres of land and upgrading wastewater treatment plants and controlling stormwater runoff. D.E.P. has built strong partnerships with organizations in the watershed and with the watershed communities and these partnerships have been a critical part of the city’s active watershed program.