Innocent passage is a concept in the law of the sea that allows for a vessel to pass through the territorial sea (and certain grandfathered internal waters) of another state, subject to certain restrictions. The United Nations Convention on the Law of the Sea Article 19 defines innocent passage as:
Underwater vehicles like submarines are required by the treaty to surface and show their flags during innocent passage.
According to the United Nations Convention on the Law of the Sea, a nation's internal waters include waters on the side of the baseline of a nation's territorial waters that is facing toward the land, except in archipelagic states. It includes waterways such as rivers and canals, and sometimes the water within small bays.
In internal waters, sovereignty of the state is equal to that which it exercises on the mainland. The coastal state is free to make laws relating to its internal waters, regulate any use, and use any resource. In the absence of agreements to the contrary, foreign vessels have no right of passage within internal waters, and this lack of right to innocent passage is the key difference between internal waters and territorial waters. The "archipelagic waters" within the outermost islands of archipelagic states are treated as internal waters with the exception that innocent passage must be allowed, although the archipelagic state may designate certain sea lanes in these waters.