That is an interesting and risky idea! The majority of the earth is water in comparison to the land, and we still do not completely know everything about the depths of the oceans. If there could be fully sustainable, floating oceanic cities; it might also drive the impetus to finally uncover the depths within them.

There is some room for uncertainty though as tectonic plates have been known to rise and fall with changes happening to the ocean periodically. So its risky for the cities involved due to tsunami's, whirlpools and water-spouts. If those cities could sail AND fly then the technology could be be employed in extraterrestrial arenas.

A lot of this sounds like it is already in the beginning phases under Ocean colonization with projects like Very large floating structures, Freedom Ship and theoretical havens operating outside government territories called Sea-steading.

I'm not entirely sold on the premises of this movement though despite offering some initial relief to scarcity of land. Maybe if they more thoroughly ironed out the pitfalls of human dwellings in general through simulated modalities, then a movement towards the ocean could be ideal.