water oak vs oak tree

water oak and oak tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
water oak	Yes	No	No	No
oak tree	Yes	No	No	No

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

water oak: relatively tall deciduous water oak of southeastern United States often cultivated as a shade tree; thrives in wet soil
oak tree: a deciduous tree of the genus Quercus; has acorns and lobed leaves

Other hypernyms of water oak include oak.

water oak (noun)	oak tree (noun)
relatively tall deciduous water oak of southeastern United States often cultivated as a shade tree; thrives in wet soil	a deciduous tree of the genus Quercus; has acorns and lobed leaves