Would it really be remotely cost-effective to take a skyscraper and refit the bottom three stories to be underwater? Sounds like a stretch. It would probably be cheaper to abandon low-lying areas and demolish the buildings there.
These things are all anchored into bedrock anyway, so it's not like you should have to worry about the material under the foundations shifting or eroding.
Part of good literature and movies is being sold the story so as to suspend disbelief. This has always happened with KSR's books for me. Funnily enough, it didn't happen with Breaking Bad - I couldn't finish it.
Anyway, diversity makes the world an interesting place!
A planet with a huge human population and associated infrastructure is way more fragile. If you threw a lens in front of Earth, all sorts of frightening things would happen to circulation in the air and seas. In the books, Mars got some batshit crazy dust storms in the middle of its process, and Mars' atmosphere had wayy less energy capacity than Earth's at that point.
I guess my point is this didn't hurt my suspension of disbelief. Presupposing the orbital lenses are possible, it's not utterly insane to conceive them as imprecise tools that would make a mess of a full fledged biosphere.
The same reason the future is so hard to predict is the same reason outcomes that don't necessarily make sense at first should be given the benefit of a doubt and reasons should be looked for, not discounted.
Not only does it make things more enjoyable, but you end up learning a lot more. It does take admitting you may not be the genius you thought you were though, so some people have trouble with that...
It's obviously fatuous to suggest that, oh no, 4 degrees of global warming is a huge problem but also, given the ability to finely control global solar influx with essentially infinite variability (they are, after all, suggesting that they'll use their sunshade to give Venus a day-night cycle, so they can open and close the damn thing every 24 hours indefinitely), you can't find a solution to Earth's global warming problem that's better than four degrees of global warming. A perfect solution? Perhaps not! But better than their status-quo.
And what was their amazing solution? Literally air-drop animals everywhere. Think that might have some unintended consequences?
The whole thing was absurd. KSR seemed to have no ability to stitch together terrestrial problems that were plausible within the framework of the rest of his solar system.
In truth, I'll probably read the whole thing eventually, since I bought the paperback already, but it's certainly not a page-turner.
Though that calls into question the premise of the book, because they would have likely either filled in the land or built levees sometime in the half-century or so that the sea was rising.
(Flooding them could never happen, but sealing them against 50 feet of sea level rise seems like a comparatively minor feet.)
If your goal is to protect all five boroughs of NYC, you have to protect Brooklyn, which faces the open sea directly. So a huge seawall south of of Brooklyn, crossing over to Sandy Hook. That looks to be closer to 10 km straight across.
The east is also tricky, because the Bronx reaches so far north and east. Your best bet is probably a dam at Sands Point, but it will have to be a good 4 km long. That should be fun.
Finally there is the Hudson river, which flows north to south. You are going to have to divert that somewhere, or it will just pool up in New York harbor until it finds a way to the sea. A Yonkers-New Rochelle canal perhaps?
If this is your plan, you will have the gratitude of an entire generation of civil engineers, construction workers, and financiers. Your taxpayers may be less enthusiastic.
https://en.wikipedia.org/wiki/Saint_Petersburg_Dam to compare to - 15mi of dams built for a much less wealthy city.