just like many elites have access to DUMB's,
Maxwell's crew probably also had access to undersea sub bases like in places like Whidbey Island, the Caribbean, Antarctica, Greenland and more.
I think 'TerraMar' was the idea that for a fee, Maxwell would bestow on you the pretended ownership of a certain lat/long coordinate at sea. And since TerraMar had not extradition treaties with anyone, maybe there was the idea that if you just got out to a TerraMar base or coordinate, you were beyond the reach of the law.