Sue,
WAY back when, a then co-worker (and now current friend) actually started a novel about NYC - in particular, Manhattan Island (Pop 1.5 milion 22 square miles) getting cut off by terrorists (we stopped right after the first WTC bombing - hit a tad too close to home)

The estimates we could come up with is that the stores would be stripped bare somewhere between 48 and 72 hours. The outer boroughs (Brooklyn, Queens, Staten Island) would do somewhat better, and ironically, the Bronx (which IS attached to the mainland) would do worse, simply because I think there are 1-2 supermarkets in the whole Bronx (at the time there were NONE). The Bonx is basically fed by corner markets (Bodegas) and small markets that would have been called a supermarket years ago. Most of the supermarkets were burned out during the blackout of 1977, and never returned

Water is another story - NYC does fairly well on water - most of it's water comes in deep "pressure tunnels" - there are 3 (the third is about 50% finished, but carrys water to at least 2 of the boroughs already - the largest non federally funded public works in history). One GOOD thing, as I've pointed out before, at least at street level, NYC water is NOT pumped, but is gravity fed, and depending on the neighborhood, will/can pressure feed to about the 4-5 floor. The PROBLEM is buildings taller than that - they must (and do) have their own water tanks - with varing capacities vs pump sizes for the building - bigger the tank for the size of the building, the lower the pump capacity to keep the tank topped off - remember that the pump needs to be able to handle peak flow over the time needed to drain the water tank, not absolute peak flow. I was involved in providing Radio Comms during the 2003 blackout. There was only a few buildings that totally lost water, and if the residents were willing to walk to street level, water was available. The big issue then becomes people who CAN'T walk down, in buildings where people won't help each other.

One "funny" issue that happened at one building I went to - it was oh - 6 hours after power was back, and they were just getting water - it seems that a LARGE percentage of the residents of that building turned on their water faucets/tubs/showers etc and left them on to "Know when water came back" - of course, we now have a greater than normal peak flow condition, with an empty water tank - everyone was getting a trickle, and the tank could not fill because the pump alone could not keep up with demand. Part of what we had to do was help management go door to door and convince folks to turn off their faucets/showers etc for about 60 minutes, so they could get a part tank of water up on the roof