Aquatic city
An aquatic city was a city that existed primarily or entirely on and/or under the surface of a large body of water such as an ocean.

The Aquans, water-breathing inhabitants of the planet , lived in an aquatic city, notably called the Aqua-City. ()

New Martim Vaz was an aquatic city in the Atlantic Ocean on Earth in the 24th century. ()

See also 
 Xanthan floating bazaar
Settlement types