Dan,
I store water, but not as much as I should, I have 4 gallons of "do not touch this" water, and usually have between 1 case, and 2 cases of bottled water - when I get down to approximately a case, I hit Costco. Add in the fact that every Saturday I tend to pick up 3-4 gallons of Fruit juice, and around 8 2L bottles of my favorite diet fizzy stuff...

One GOOD thing about NYC, the water supply (unlike many cities) does NOT rely on power, so it will be very hard to take out (amend that - if you are above about the 5th floor, you need power to pump the water up)

Unlike Manhattan, I know where there is a local spring - I would NOT want to drink it long term, but I have seen the water quality reports, and I would drink it if desperate, and with a good filter....

Manhattan Island never had much in the way of a water supply, which is why you will find they were one of the first cities in the US to put in a serious municipal water system. Once the link of Bad water/cholera was made, NYC spent a LOT of Money a good history is here - note the first resevoir - 1776!! http://www.nyc.gov/html/dep/html/drinking_water/history.shtml The Croton system goes back to 1842! (and is still in use!)

If it's going to be a water problem for more than 2-3 days, I'm bugging out, and I know where I'm going - LOTS and LOTS of fresh water