water moccasin vs pit viper

water moccasin and pit viper both are nouns.

Word	Noun	Adjective	Verb	Adverb
water moccasin	Yes	No	No	No
pit viper	Yes	No	No	No

As nouns, pit viper is a hypernym of water moccasin; that is, pit viper is a word with a broader meaning than water moccasin:

water moccasin: venomous semiaquatic snake of swamps in southern United States
pit viper: New World vipers with hollow fangs and a heat-sensitive pit on each side of the head

water moccasin (noun)	pit viper (noun)
venomous semiaquatic snake of swamps in southern United States	New World vipers with hollow fangs and a heat-sensitive pit on each side of the head
any of numerous North American water snakes inhabiting fresh waters