La Convenzione di Montego Bay stabilisce che ogni Stato è libero di stabilire l'ampiezza delle proprie acque territoriali, fino a un'ampiezza massima di 12 miglia marine, misurate a partire dalla linea di base (articolo 3 Convenzione di Montego Bay

Standards are fairly clear. Who "owns" what is ambiguous and often disputed (e.g., South China Sea, Spratleys, etc.). The land claims determine the maritime ownership.

The maritime limits of of an nation are usually governed by one of the only truly international legislations, known as UNCLOS (United Nations Convention of the Law of the Sea). It defines 3 different limits, namely:

Territorial Waters: Up to 12 nautical miles from a country's agreed coastal "baseline" extending seawards. Most countries can exercise their outright (national) jurisdiction in all matters only until this limit. Anything outside this limit falls actually within International Waters.

Contiguous Zone: extending 12 nautical miles seawards from the Territorial Waters, i.e. 24 nautical miles from the baseline, within which a country can exert "limited" jurisdiction with respect to its customs, immigration or pollution laws.

Exclusive Economic Zone (EEZ): extends upto 200 nautical miles from the baseline. Mainly concerned with the exploratory limits for living and non-living natural resources (oil/gas/fishing) to which a country has sovereign rights.

The definition of "Baseline" can differ depending on the area in question, but in most cases it is quite close to the actual coastline (referenced to the visible coast at low tide).

However, like any other legislation of international significance, UNCLOS is fraught with certain inconsistencies and disagreements, in certain areas.

You may refer to below link to give you an idea of what the situation is:

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/maritime-boundary#:~:text=Territorial%20sea%20is%20defined%20under,and%20air%20space%20above%20it.