It also ignores the (not exactly minor) problem that, as their owners can attest, a boat is a hole in the water into which one pours money.
There are some commercially viable things done on boats (fishing, offshore drilling, etc.) and some recreational ones; but few things done on land get cheaper when done on water; unless you have in mind some straw man comparison between costs in some ultra high end urban center and the scungiest refurbed cargo ship you can get your hands on.
They are welcome to try, of course, it's their money; but I've yet to see a 'seasteading' plan that doesn't appear to be a fairly uncomfortable yacht club.