Nah. Fentanyl is potent. But it's not that potent. Think about the HUGE amounts of water it would be diluted into.
Not enough info to know for certain. But here's my guess.
The lethal dose of fentanyl is about 2 milligrams.
https://www.oxfordtreatment.com/fentanyl/lethal-dose/
The daily water use of NYC is about 990 million gallons. Call it 1 billion.
https://data.cityofnewyork.us/Environment/Water-Consumption-In-The-New-York-City/ia2d-e54m/data
Suppose they have about a day's worth of water in the water-mains, tanks, and pipes.
So if you dumped 1 metric ton (1 billion milligrams) into the water supply it would have a concentration of about 1 milligram per gallon. You'd have to drink 2 gallons of water to get a lethal dose. Maybe if you dumped several tons in at once then you might kill a few people before they figured out what was happening and word got out.