Article I, Section 10, Clause 3:

No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.

Article I, Section 10, Clause 3, prohibits states from interfering with interstate and foreign commerce by imposing duties of tonnage—charges to access a port based on a vessel’s capacity (i.e., its tonnage)—without congressional consent. States may impose other types of taxes or charges on vessels provided they do not constitute duties of tonnage or otherwise violate the Constitution.