In which case, how do they get to the seastead? It seems that they would need a ferry, but does that ferry port have passport facilities and act as an international port? And if you're on the seastead for a long period of time and US access required a visa, how do you now renew that visa seeing that you couldn't land in the US without it and there is no US Embassy on the seastead?
Or more plainly... what problem is this solving for whom? It doesn't seem to solve the problem for anyone who is unable to get into the USA without a visa.