I’ve been casually enthralled by such projects since http://oceania.org - building a new nation on artificial land is an attractive concept*.
Unfortunately, cost of living is surprisingly high in such places - made apparent by the grinding poverty occupying most natural islands of comparable size. It would require a “Galt’s Gulch” scale effort by the very wealthy & hyper productive to make work for any meaningful period.
Those contemplating such efforts might do well to consider something far cheaper and easier to access: one can easily find land “out West” at $1000/acre; that’s a whole lot cheaper than the $1000/_foot_ you’d be looking at to build & occupy a viable floating city.
There’s also places like China’s “ghost cities” where entire literal cities have actually been built, with practically nobody living in them. Would be a whole lot easier & cheaper to just arrange occupancy there, than to build it yourself starting with making the very ground to build upon.
But then again, I’ve noticed there’s good money in proposing “mega projects” - not necessarily actually building them (though that’s a great bonus if you can pull it off), but pitching a good proposal to the ultra-rich can get you enough $$$ for a small team for a few years to research the possibility and make impressive presentations to your patron’s swimming-in-wealth friends.
To wit: they’re neat, but I don’t see “floating cities” happening. There’s too many cheaper & more practical options available.
* - much later I discovered that the US military built & occupied an artificial island. The nearby political shift some years later resulted in failure to continue regional recognition of its ownership/independence, resulting it in being shelled and the occupants vacating. The artificial island was eventually retaken by its original builders, but at a staggeringly high human cost.
I would think that for the super-rich this is equivalent to a gated community with superlative views. Can’t imagine them moving to the cheap land unless they could buy up most of it to get isolation, if not a view. I imagine such a community could afford a pretty good anti-pirate defense force. You could even hire local sharks and brag about saving the sealife.