Not too keen on the water theme, sounds like either Bioshock, or a lot of fast traveling, you've brought quite a few good elements to the table though. I wouldn't mind a sunken area thats filled with water, and to get to the civilized areas under the water, you'd need a rebreather. NYC is one I like the sound of, but Naughty Dog is developing "The Last of Us" which is set in post apocalyptic New York. Sorry I don't have anything to add, I'm more of an evaluator. I'll think about it though, and post when I do get some.