Can’t see any problem with VFR off coast outside 200 mile limits, as long as said flight didn’t interfer with any other activities ( other VFR flights, drilling, shipping ) .
If the sonic boom, as an example, blew out the windows of a container ship or fishing trawler, then there would be issues of national relations that diplomats would have to resolve.
My reference to the South Atlantic was made mainly to emphasize that today, you would have to go far to get away from civilization so that you fun didn’t effect others. Another good place to fly mach 1 plus would be the Canadian Arctic.
Come to think of it, this discussion is almost hypethetical. In terms of total civial aviation, the number of privately owned planes that can travel super sonic is very rare ( most private jets and warbirds are sub sonic, F-86 in a dive an exception). Of the the few that do exist, like F-100’s or F-4’s, these are used by companies / contractors who work for the DoD. Plus the cost of maintaining and flying such a machine would limit its truely private ownership to the very, very very wealthy.