Suppose for a moment that you have a "starter" seastead, Seatopia, of several platforms and 2000 people that is independent from any existing government. How is Seatopia going to remain independent? You're not included in any existing intergovernmental agreements, treaties, or trade organizations. 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.
I have heard it argued that the impact of American trade restrictions on Cuba is quite small, since trade has a marginal value. I.e. Trade with the first country is very valuable, trade with the 50th country isn't so important.
Considering how many economists seem to dismiss the power of punitive trade restrictions, that makes me very suspicious about your claim that it could have significant costs. At least unless it involves several countries (such as a UN agreement).