Recently the number of bays bordered by more than one State has increased owing to the break-up of existing composite States. The legal regime of such bays thus merits particular attention. In this regard, a question arises as to whether States bordering a bay may draw a closing line across the mouth of the bay. Two different views can be identified.
According to the first view, the coastal States bordering the bay may draw a closing line by agreement. In fact, the 1988 Agreement between Tanzania and Mozambique closed the Ruvuma Bay, by drawing a straight line linking two cross-border points. Article II of the Agreement provides: ‘All waters on the landward side of this line constitute the internal
waters of the two countries.' Under the same provision, the internal waters are apportioned by means of a median line.
According to the second view, the normal baseline rule should apply to bays bordered by more than one State because such bays are not regulated by Article 10 of the LOSC or historic bays. In this view, bays bordered by more than one State cannot be closed by a line across the mouth, and the low-water mark around the shores of the bays constitutes the
baseline. Legally speaking, the waters of a closing line of a bay are internal waters under territorial sovereignty. As territorial sovereignty is exclusive by nature, the internal waters of one State cannot belong to another State at the same time. Thus, the idea of a bay bordered by more than one State, the waters of which are internal waters, contains a conceptual contradiction.
In the 1992 Land, Island and Maritime Frontier Dispute, the legal status of bays bordered by more than one State – the Gulf of Fonseca – was discussed in connection with historic bays. In this case, the Chamber of the ICJ held that the Gulf was a historic bay, the waters whereof were held in sovereignty by El Salvador, Honduras and Nicaragua. However, Judge Oda questioned this view and argued that the waters of a historic bay are nothing other than internal waters, and that these waters of one State cannot abut the internal waters of another State. The learned Judge took the view that ‘there did not and still does not (or, even, cannot) exist any such legal concept as a “pluri-State bay” the waters of which are internal waters'. According to this view, apart from the landward side of the low-water mark, the waters of bays bordered by more than one State fall within the category of the territorial sea, the EEZ or the high seas.
A particular issue arises where a judicial bay is to be divided into plural States because of the dissolution of one State. This issue was discussed in the 2017 Croatia/ Slovenia Arbitration. According to the Arbitral Tribunal, the ‘Bay of Savudrija/Piran' (Croatia) or ‘Bay of Piran' (Slovenia) was established as a juridical bay. However, subsequently the bay had two coastal States as a result of the dissolution of the Socialist Federal Republic of Yugoslavia. In this regard, the Tribunal took the view that the dissolution did not have the effect of altering the acquired status and that the bay
remained internal waters. As two coastal States cannot exercise territorial sovereignty over the same areas at the same time, delimitation of the bay was needed. While the LOSC contains no provision on the delimitation of internal waters, the Arbitral Tribunal, in the Croatia/Slovenia Arbitration, considered that delimitation within the bay is to be made on the basis of the same principles as are applicable to the delimitation of land territories. It thus made the delimitation of the bay on the basis of the effectivités at the date of independence.
source: The International Law of the Sea, Yoshifumi Tanaka