polynya vs body of water

polynya and body of water both are nouns.

Word	Noun	Adjective	Verb	Adverb
polynya	Yes	No	No	No
body of water	Yes	No	No	No

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

polynya: a stretch of open water surrounded by ice (especially in Arctic seas)
body of water: the part of the earth's surface covered with water (such as a river or lake or ocean)

Other hypernyms of polynya include water.

polynya (noun)	body of water (noun)
a stretch of open water surrounded by ice (especially in Arctic seas)	the part of the earth's surface covered with water (such as a river or lake or ocean)