https://en.wikipedia.org/wiki/Underwater_habitat
Humans don't need more room, they need more watered plowland. And the undersea habitat provides nothing for that, because they are too deep for the photosynthesis.
Even China is mostly a desert, and Japan has large empty areas as well.
Say, a megalopolis with 1000 humans/km2.
Say, an average ceiling height ~4 m2.
It's 4 * 1 000 000 / 1 000 = 4 000 m3/human.
8 bln humans * 4 000 m3/human / 100 m height / 1 000 000 m2 in km2 ~= 320 000 km2.
sqrt(320 000 *4 / pi) = 640 km2 in diameter.
The whole humanity could be easily placed on Sumatra
https://www.britannica.com/list/the-largest-islands-in-the-world
But what to eat there?