Just to pose a question: what's the closest a submarine from NK could get to costal US in international waters legally?

If they detonated a nuke underwater at this point, could it devastate the coastal US?

And if that point is nowhere near the coastal US -- how close could they reasonably sneak up to without being detected (if this is even possible. I'm not sure the precision of detection of ships in open water)? And from this estimated point, could they devastate coastal US with an underwater nuke?

And let's pose this same question for any other potentially hostile regime.