This would be the United Nations Convention on the Law of the Sea (UNCLOS). It's interesting that the US is among the nations that have never ratified the treaty.

Also, Denmark has a long 500+ year history of charging ships to transit in and out of the Baltic Sea, so this really isn't a new concept (Sound Tolls).

All foreign ships passing through the strait, whether en route to or from Denmark or not, had to stop in Helsingør and pay a toll to the Danish Crown. If a ship refused to stop, cannons in both Helsingør and Helsingborg could open fire and sink it.