water oak vs oak

water oak and oak both are nouns.

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

As nouns, oak is a hypernym of water oak; that is, oak 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: a deciduous tree of the genus Quercus; has acorns and lobed leaves

Other hypernyms of water oak include oak tree.

water oak (noun)	oak (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
	the hard durable wood of any oak; used especially for furniture and flooring