I'd agree with that, Jesselp has put much thought into his situation.

Along the lines of water, consider a small backpack to carry stuff for the walk home. I don't know the specifics of your messenger bag, but if you need to walk home a backpack might be a better option. Then again, maybe a messenger bag is the best option in NYC. Does it have room for carrying water for the walk to your home? A couple of the new Camelbak models are bottle based rather than using a bladder for water -- I kinda like the Camelbak Sutra. I prefer to drink from a bottle than from a tube, but that's a personal preference thing.

Right now my get home backpack is a Camelbak Alpine Explorer. It's set up as a minimalist overnight pack. I have a 25 mile walk, so if TSHTF and a big earthquake out here tears up the roads or knocks down a certain bridge. . . For water I use a bottle but I have the bladder to carry additional water for the walk. Actually in the summer I might wait for the sun to go down. . . Naah, most likely I'll just hang out with my truck (food, tent, sleeping bag, camp stove, et al) and wait for the dust to settle. No need to become another casualty along the road.