In my twenties, while living in LA half a century ago, I was part of a collective called "The Island". We envisioned communities floating in the open ocean, growing kelp and fishing (among other productive activities). Family/business "hexagons" would be hooked together in a 2-D tensegrity arrangement, which could roll with the wave (and disperse if the waves become really large). The rigid elements woud be fabricated from ground up (locally grown) seashells. Solar-powered refrigeration would continuously provide the ice to make the hexagons unsinkable. Etc. etc.

Joining the world's 600,000 natural islands, 10,000 such "pelagic islands" each with 10,000 inhabitants would accommodate 100,000,000 people. Scale up both those figures by 10 and you have 10 billion. Not 100 billion, but as one comentator noted: "What's the attraction of 100 billion anyway??"