high sea vs international waters

high sea and international waters both are nouns.

Word	Noun	Adjective	Verb	Adverb
high sea	Yes	No	No	No
international waters	Yes	No	No	No

As nouns, high sea and international waters are synonyms defined as:

high sea and international waters: the open seas of the world outside the territorial waters of any nation

high sea (noun)	international waters (noun)
the open seas of the world outside the territorial waters of any nation	the open seas of the world outside the territorial waters of any nation