water nymph vs naiad

water nymph and naiad both are nouns.

Word	Noun	Adjective	Verb	Adverb
water nymph	Yes	No	No	No
naiad	Yes	No	No	No

As nouns, naiad is a hyponym of water nymph; that is, naiad is a word with a more specific, narrower meaning than water nymph:

water nymph: (Greek mythology) any nymph of the water
naiad: (Greek mythology) a nymph of lakes and springs and rivers and fountains

Other hyponyms of water nymph include sea nymph.

As nouns, water nymph and naiad are synonyms defined as:

water nymph and naiad: submerged aquatic plant having narrow leaves and small flowers; of fresh or brackish water

water nymph (noun)	naiad (noun)
(Greek mythology) any nymph of the water	(Greek mythology) a nymph of lakes and springs and rivers and fountains
a fairy that inhabits water	submerged aquatic plant having narrow leaves and small flowers; of fresh or brackish water
submerged aquatic plant having narrow leaves and small flowers; of fresh or brackish water
a water lily having large leaves and showy fragrant flowers that float on the water; of temperate and tropical regions