Internal water is the waters and waterways on the landward side of the baseline from Thailand's territorial sea (UNCLOS Article 8, paragraph 1). It includes bays, rivers estuaries, lake, etc. The coastal nation has sovereignty over the internal waters (UNCLOS Article 2) as the sovereignty over the territory. Therefore, foreign vessels must have permission for the state before passage within internal waters. There are five areas include:
Area 1, called Ao Prawatsat, covers all area in the inner Gulf of Thailand which is over the determined baseline as its boundary.
Area 2 includes the area of Lam Ling through the Thai-Cambodian border.
Area 3 includes the area ranging from Lam Yai to Lam Natam.
Area 4 includes the area ranging from Ko Phuket to Thai-Malaysian border.
Area 5 includes the area ranging from Ko Kong Ok, located southward of Ko Samui which is one of the reference points for the Area 2’s boundary, through Ko Kra, Ko Losin, and meets the coast at Thai-Malaysia border at the Kolok estuary, Narathivat Province.