The skyline would become an eerie monument to the dead.
The Manhattan borough of New York City is a particularly interesting case study given that it has approximately 1.6 million people who live there and typically sees its population double during a work day. That’s three million people, a majority of which are going to be in those buildings.
If this flood happens in less than twenty four hours, you’re not going to see many people successfully leaving the city. That means you’re looking at millions of people potentially stranded in buildings, assuming they’re lucky enough to be in structures higher than five to ten stories depending on location.
There will be no power. The available food will probably be limited to what white-collar workers brought in for lunch. You are now looking at millions of people stranded without the proper resources to survive. Lots and lots of people are going to die inside these buildings.
Now consider the bigger picture. The image below shows areas likely to flood at a variety of possible future sea levels.
What’s somewhat surprising about this image is just how much of a sea level rise you would need to actually flood the entirety (or even just the majority) of Manhattan.
If this kind of sea-level rise occurred in here you can be sure it’s happened everywhere else. Now you’re not only dwarfing the hundreds of thousands (if not millions) of casualties in Manhattan, but you’re making it extremely unlikely that there will be any rescue attempt to save the survivors.
Long term, you now have over two hundred steel graveyards reaching into the sky. The people trapped in the buildings are not going to live long enough to become a culture. And attempting to move into them afterward means you not only need to build the infrastructure to survive, but you need to deal with the millions of bodies decomposing. In case that wasn’t troubling enough, seawater is very corrosive. These buildings were not constructed with the intention of remaining structurally sound when submerged in seawater for extended periods of time.
In the event of a disaster on this scale, New York City would finally get a chance to sleep.