high sea vs body of water

high sea and body of water both are nouns.

Word	Noun	Adjective	Verb	Adverb
high sea	Yes	No	No	No
body of water	Yes	No	No	No

As nouns, body of water is a hypernym of high sea; that is, body of water is a word with a broader meaning than high sea:

high sea: the open seas of the world outside the territorial waters of any nation
body of water: the part of the earth's surface covered with water (such as a river or lake or ocean)

Other hypernyms of high sea include water.

high sea (noun)	body of water (noun)
the open seas of the world outside the territorial waters of any nation	the part of the earth's surface covered with water (such as a river or lake or ocean)