Since you aren't talking about the "political, legal and financial" [which would be the most important/critical factor], why not house people on the "humungous" oceans of the world.

Or maybe, you could include "political, legal and financial". Since some govt in this world are foolishly planning to reduce CO2 and spend huge globs of money doing so. "Take" some small percentage of this budget to make a floating island. The island would suck up CO2 by designing it so coral is grown to become part of floating structure.
So project would start by doing research on how you would make this "floating coral reef". Then make some small coral farms, which can then be combined, and add humans living in the eco-system.
This could lead to a world where a large percent of people could live on the beach.