high sea vs main

high sea and main both are nouns.

Word	Noun	Adjective	Verb	Adverb
high sea	Yes	No	No	No
main	Yes	Yes	No	No

high sea (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

high sea (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