As you note “live on a boat” is substantially more of a collective action problem than “live on land” is(negative externalities and violence and such are obviously present in both cases; but it’s less often the case that the entire neighborhood sinks if one person neglects lawn maintenance).
What I’m not really sure of is how many of the proponents of the idea are just somewhat untutored in the ways of boats; how many take a “Captain’s word is law is pretty cool if you plan to be the captain” view and are just hoping to build a small pond where they don’t have to contend with incumbent big fish with standing armies; and how many take the “but it’ll all be based on contracts and the sort of free association encouraged by being surrounded by ocean for miles in every direction; so it won’t be state coercion!” position.
I’m pessimistic about things working out for any of them; the economics of boats are such that they mostly get used either for profitable activities that can’t be done on land(fishing, bulk shipping, marine oil/gas extraction, etc.) or for the amusement of people who are bringing the money in from outside. In the former case, I’m having a hard time thinking of things that a hypothetical seastead has a comparative advantage in: if it can be done on land, doing it in a reasonably cheap location will cost a lot less than doing it on a boat, even if you don’t score some tax breaks or subsidies, if it cannot, you are competing with the existing marine operators of the world(possibly on a ship that isn’t even welcome in most ports) who have less incentive than you to make their ships comfortable rather than merely endurable for the duration of the job.
As recreation, the numbers work out much better; but at the cost of making your social experiment little more than a libertarian yacht club(which is fine, probably much more interesting than a normal yacht club; but useless if you want to prove an alternative way of doing things, rather than simply demonstrating that almost any silly idea can be made to work with enough subsidies).