It's also asking residents of some Ocean County communities to voluntarily limit outdoor water use.
Water company spokesman Richard Barnes said there's been a significant spike in water use since the weather turned hot again late last week.
Suffolk County is also on a water alert, and East End customers are asked to reduce water usage. The Riverhead Water District asked residents and businesses to cease lawn irrigation until the heat wave breaks.
Residents in New York's Rockland County are also being asked to conserve water.
United Water spokesman Steve Goudsmith says "the heat wave is testing the limits of our supply."
The company is asking its customers to restrict water usage outdoors for the next three days. It says that includes outdoor sprinklers, which could increase water usage by 50 percent.
Goudsmith said the restrictions should be lifted once the temperatures come down.