international waters vs main

international waters and main both are nouns.

Word	Noun	Adjective	Verb	Adverb
international waters	Yes	No	No	No
main	Yes	Yes	No	No

international waters (noun)	main (noun)
the open seas of the world outside the territorial waters of any nation	a principal pipe in a system that distributes water or gas or electricity or that collects sewage
	any very large body of (salt) water

international waters (adjective)	main (adjective)
	(of a clause) capable of standing syntactically alone as a complete sentence
	most important element
	of force; of the greatest possible intensity