“We always ask,” he said. “Otherwise, it’s a waste. We have to pay for the water they use. The less we use, the less we have to pay for. And even though New York City has the best water, bottled water is a selling point, and you can make a decent profit.”

The city’s Department of Environmental Protection dates the regulation to at least 1991, when the city was facing a drought emergency, but it has barely been enforced. In 2002, during another drought emergency, 14 warnings were issued against restaurants. There is no record of a summons being issued during nonemergencies.

“Someone’s got to scrub those law books down,” Mr. Costentino said.

Which is exactly what Mayor Michael R. Bloomberg’s administration did in its final days. Except when drought emergencies are declared, the regulation barring waiters from reflexively serving water was repealed by the administration. The notice of the adoption of the final rule was published in the City Record on Jan. 23 and will take effect 30 days later.

“We’re very happy that the administration is taking a glass-is-half-full attitude to reviewing old regulations,” said Andrew Moesel, a spokesman for the New York State Restaurant Association. “It’s good for both the city and the industry to get them off the books.”

The repeal of Prohibition in 1933 — 13 years after it went into effect — touched off binge drinking and wild parties, but this time restaurant patrons are unlikely to even notice. That is because restaurateurs have always had an incentive to sell soft drinks and alcoholic beverages instead. Also, since more accurate metering took effect in the last decade there is more of an incentive to consume less water.

If it seems anomalous that a city intent on saving water is repealing what seems like a conservation measure, chalk it up to expediency. Restaurants, after all, are not obligated to automatically pour water.

“There’s no empirical evidence that this measure saves water outside of a drought emergency,” said Christopher Gilbride, a spokesman for the Environmental Protection Department. “But because they’re paying for the water they use, they have an incentive not to serve unless asked.”