That means that any nation that doesn't like your experiment can impose punitive restrictions on trade with Seatopia at trivial cost to its domestic economy but very significant costs to Seatopia.
For that to have any meaningful effect on a small nation you would need every country on earth to put sanctions on seatopia. Good luck convincing Cuba not to trade with them. Even if 100% sanctions were to happen, they never work. How's that embargo on Mexican Heroin going?
I think that Mexico could be embargoed/blockaded quite effectively and cheaply if it were a few oil platforms in the middle of international waters instead of a large nation sharing thousands of miles of border with the US. I am assuming that Seatopia is going to offer legalized/deregulated services not found elsewhere, since A) they have little else in terms of comparative advantage for the domestic economy and B) if other nations offered what Seatopia does, people would just go there instead of to a platform in the middle of international waters. If Seatopia dares to live the libertarian dream -- no legal restrictions on weapons, drugs, gambling, prostitution, or private banking, and zero tax rates for everybody -- that's plenty of incentive for the IRS, DEA, and DHS (and most other nations' tax systems, drug enforcement, and counter-terrorism agencies) to shut down Seatopia.