So this will probably ramble a bit. I've lived on a boat before and plan to retire to live on a boat when my (too) fast-approaching old age hits me. I've done quite a bit of research and looked into life on the sea.
Here's the issue with SeaSteading: boats are expensive. Not only are they expensive to build, they're maintenance hogs that need about 10%* of their construction cost each year in maintenance. Further boats have relatively tiny amounts of space. A boat can quickly feel like a prison if you can't get off, not only is the boat's cabin substituting for your house, but it's deck is basically substituting for your yard and it's galley is not only your kitchen but also the local grocery store since it needs to store everything you'll eat for [duration of voyage]. Regular shore visits are required for most people to feel comfortable which is fine for people who just want to laze around and sail in their retirement years but doesn't work if you want a sovereign boat nation.
Nor do boats offer amazing amounts of freedom. Yes, they sound like it and yes, having no neighbors and no other humans from horizon to horizon is an awesome and humbling feeling and I love it. But you also have to be ludicrously disciplined for that lifestyle. Don't like the layout of your room? Tough, it's part of the structure, should have planned better. Like knicknacks? Tough, you don't have room for them and in fact have to get rid of everything except absolute essentials because space is limited, power is limited, water is limited, space in your blackwater tank for your own waste is limited, and unless you want to throw your garbage in the sea you've got to watch that lest your boat stink and fester and biblical-scale mold or insect plagues attack you. Even bathing is troublesome when you have to ration every drop of fresh water so don't plan on swimming too much because you need to wash the salt off your skin.
Ultimately boats cost more for less space than land. Floating cities are ultimately just really slow boats, they still need all the expensive infrastructure. Generally speaking the boat life is for the wealthy and/or retired who enjoy that life. In order for a hypothetical floating city/fleet to operate, it has to overcome the innate cost disadvantage it has vs. land.
I see it commonly put forward that outside of restrictive national laws, the fleet will attract geniuses who's brilliant discoveries will pay for the fleet. But this presumes that a large number of geniuses want to live on a ship with it's attendant risk of pirates and storms and cramped conditions, rather than a nice college campus where the labs have acreages of space rather than square footages. It also presumes that every nation on the planet is restricting their own research for no apparent reason, and that further they fall into a very specific level of restriction where every nation in the world is too squeamish to do their own research, but not too squeamish to pay the fleet geniuses for said research and certainly not squeamish enough to send a naval force to stop whatever Dr. Mengele experiments they have going on. These thought experiments often revolve on the value of being away from the restrictive laws of land-based nations without recognizing that due to every last square foot and rivet needing to be in it's proper place and every drop of water and crumb of food being measured, boat life gives you far less freedom in day-to-day living than land life does. Granted being able to sail around at will is nice but as a scientist living on a massive research ship, you don't actually have that freedom, whatever complex council governs the ship does.
Ultimately you have to find a resource to exploit that exists on the sea and not on land. Fish are obvious but fishing boats already do this, and better than the hypothetical fleet because fishing boats don't need to blow tons of space on sufficient living quarters and entertainment facilities to be comfortable to live on. Research is okay as long as you're specifically researching oceanic phenomenon but governments already have research vessels so you have to find some advantage over them. Undersea mineral deposits are possible but will be exploited by land-based mining companies faster than sea-based mining companies can do it, since the land-based companies can access vast factories to build their mining equipment on land (and this already happens with offshore oil rigs).
In the end there's pretty much one thing the ocean has that you can't get on land, and that's ocean. Needless to say tourism is the biggest reason to live on the sea with mariculture second, and, in fact, oceanic cities already exist (and are quietly ignored by the people saying they want to build an oceanic city).
Seaventures, for instance, is an abandoned oil rig that was converted into a luxury hotel with diving tourism as it's main draw. There's also
Sealand, a sovreign nation built on an abandoned fort slightly outside British Territorial waters which has a hilarisad history of trying to be an indepedent nation and constantly getting attacked by pirates (and at one point nearly bought by The Pirate Bay) taking it over despite being in the middle of the EU where you'd think that wouldn't be happening.
I have seen one decent proposal for a real large-scale floating city, the author proposed powering it with OTECs and then using the vast power an OTEC can potentially generate to create hydrogen which would be shipped to shore for sale, while also using the nutrient-rich deep-sea water the OTEC brought up to grow various algae and shellfish in farms on the lower levels for export. It was actually a fairly decent piece of science fiction, though there's a few breakthroughs needed and in particular, the kilometers-long pipes needed to power an OTEC are still ruinously expensive (and need a mountain of work to make sure they aren't promptly ruined by all manner of sea life growing on and clogging them). Still significantly more viable than assuming geniuses want to live on boats by preference, though there's still little reason not to build the OTEC on shore and do the mariculture there instead of trying to build a city somewhere miles from land.
*That's a vague rule of thumb, older boats will want more as their parts wear out and some designs are obviously more maintenance-intensive than others.