The New Croton Reservoir supplies much of New York City's tap water, and people are allowed to — and do — go out on boats and fish. I'd venture a guess that almost every Saturday and Sunday, a piece of bread falls into the water.
I don't know how long it takes for the water to reach New York, but strongly suspect that the water at the tap in New York is connected by water to the reservoir. (This last suspicion is amplified by the fact that the water travels to New York under gravity alone. For it to do so implies that there's no "break" in the water: any break would be filled by the water propelled by gravity.)
Even a small amount of chametz that gets mixed into water on Pesach makes the water chametz.
I know some New Yorkers do not drink (or cook with) water that comes from the tap on Pesach, but most are not so stringent. Why aren't they? What heter is there to use the water? (Sources, please, if possible.)