Importing isn't the problem, exporting is. A seastead will necessarily pay a steep premium on imported goods, even basic ones.
The cost of shipping is mitigated by goods going both ways. When you are shipping to a zero-export entity such as a seastead, the receiving entity is expected to bear the cost of the return trip with empty holds.
This has happened before. Check out the history of pirate radio in Europe. The European countries responded by creating national laws that embargoed the pirate radio stations. The Spanish were never apart of it and British citizens just ignored and resupplied illegally. Decades after the embargo was passed there were still radio stations opening.
Still annoying and likely if you're successful, but probably not killer.
Why would you be importing at exorbitant fees (relative to what an island nation would be paying anyways)?
And why would they be embargoed by everyone? Not even Cuba is embargoed by everyone. Why would everyone embargo a seastead inhabited by upper income individuals? In general the market is pretty responsive to wherever the money is.
