main vs high sea

main and high sea both are nouns.

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

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

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