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.
The Seasteading Institute offers this naïve explanation as to why it won't happen:
The first seasteads will operate under the same maritime laws as existing ships... Although [cruise] companies have major operations within U.S. territorial waters, the U.S. does not interfere very much with their operations. This is due, in part, to the fact that cruise lines bring in jobs and revenue to the U.S. economy. If the U.S. were to try to interfere too much, the cruise lines would simply move their operations elsewhere. Similarly, seasteads will trade extensively with land-based businesses. The people who profit from those relationships will encourage their government not to interfere and drive away the seasteads business.
By the same logic, the United States does not and can not impose punitive restrictions on trade with Cuba or Iran. Theory says it doesn't happen because it would be economically undesirable. The theory needs some work.