anything past 3 miles away from the coast of the nation is international waters. That's why after like half a day a cruise will usually be in International waters(unless it's in small seas like the Mediterranean).
Depending on the location, Oil rigs.
Heavy storm surge will still make them move, since they aren't "Rigid" structures, and instead Flexible to a point. Those things are amazing structures. They'd probably survive far longer after the fall of humans than almost anything save Nuclear power plants, Hospitals/critical buildings, and some special structures.
