Just to add one thing, it is possible to exempt a particular situation from international law more generally by entering into a bilateral treaty with a disputing state. If Canada and the United States were to reach a bilateral agreement on the Northwest Passage, that would take it out of general international law and therefore eliminate the precedent.
That's something Canadian governments should be thinking about, given the common interest in the defence of North America, particularly at a time when the Arctic waters are opening. This doesn't need to be a precedent for anyone. There just has to be diplomacy followed not even by a treaty that requires the advice of the U.S. Senate, but an executive agreement between the two governments.