seven seas vs body of water

seven seas and body of water both are nouns.

Word	Noun	Adjective	Verb	Adverb
seven seas	Yes	No	No	No
body of water	Yes	No	No	No

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

seven seas: an informal expression for all of the oceans of the world
body of water: the part of the earth's surface covered with water (such as a river or lake or ocean)

Other hypernyms of seven seas include water.

seven seas (noun)	body of water (noun)
an informal expression for all of the oceans of the world	the part of the earth's surface covered with water (such as a river or lake or ocean)