I suspect you're underestimating the cost and complexity of such an operation. Artificial islands exists, but need constant upkeep (adding more material as it'

I suspect you're underestimating the cost and complexity of such an operation. Artificial islands exists, but need constant upkeep (adding more material as it's eroded/reclaimed by the sea). Those don't float, they're anchored to the sea bed. Things that do float need even more upkeep, because a larger part of their surface area comes in contact with saltwater, which is very good at ruining everything it touches. Ship maintenance is expensive. Floating city maintenance would be staggeringly expensive. I can imagine it being nearly as complex as building a city on the moon.