The international Tribunal for the Law of the Sea, following the United Nations Convention on the Law of the Sea is the big one, though in this case really only covers the manner one can legally seize a ship -- the how, not the why.

For that you'd refer to the International Court of Justice (specifically Ukraine v. Russian Federation ), the International Criminal Court (This case in particular. ), and the United Nations General Assembly's United Nations Security Council Resolution 2623.

And yes, those case are all linked and part of determining the legality of seizing Russian "shadow fleet" ships.