Technically this is quite possible. Modern container ships are very long, 1300 feet = often bridge 2 or more waves = the ship is a beam with 2 or more cushiony

Technically this is quite possible. Modern container ships are very long, 1300 feet = often bridge 2 or more waves = the ship is a beam with 2 or more cushiony supports that mean the full weight of the ship bridges them. Consider a square ship as long as a container = 1300 feet square, it will have to act as a beam in all directions. This is quite doable - given the will to do it. This is a quarter mile square. You can make it bigger, but it may be wise to make it as a number of quarter mile squares all connected via flexible beams to allow the tiles to flex, with enough space between for rough sea flexure. They could even be made of concrete with stainless steel rebar. They call it ferrocement https://en.wikipedia.org/wiki/Ferrocement https://en.wikipedia.org/wiki/Ferrocement making it cost a lot less than steel. They could also be made of thicker concrete with stainless steel rebar, like the floating docks of the D-Day invasion https://en.wikipedia.org/wiki/Mulberry_harbour https://en.wikipedia.org/wiki/Mulberry_harbour This would allow for long lasting floating cities, made with several security bulkheads so a hole in one section could be isolated. The entire volume within could be used for living, manufacturing, recreational spaces as well as hydroponic LED lit green houses. These would be anchored in areas with good weather(hurricane free) and no risk of hitting rocks. Many countries whose land area will vanish in the next 200 years could become entirely oceanic. They could have extensive areas of aquaculture around them? They might be moored over ocean mining areas so they could recover manganese nodules etc. - if ecologically allowed. In any event, via aquaculture and fisheries with internal green houses the only thing they would need is power. The top can be solarised, in addition thorium reactors would be viable (China is finalising this design) - we would not want to use coal/gas for power.