The city already owns those reservoirs, as well as the water tunnels that deliver the water into the city (which require little maintenance, as they run on gravity rather than pumps).
I'll admit to ignorance here. A poster above suggested NYC does not own the Rondout reservoir, another one said they do own two other reservoirs. If the city owns all of them, great. If there are some needed for the water supply, then they would have to be purchased.
Of course, all of this is on the level of fantasy, because it's not going to happen.
When it comes to natural resources, ownership /= control.
The city may have ownership, but the local/state/national governments still have say on things like environmental protection. Owning a forest doesn't give you the right to clearcut every tree. Owning a reservoir does not give you control over where that water comes from (the watershed) nor any of the environmental or safety rules governing water.
