water moccasin vs cottonmouth moccasin

water moccasin and cottonmouth moccasin both are nouns.

Word	Noun	Adjective	Verb	Adverb
water moccasin	Yes	No	No	No
cottonmouth moccasin	Yes	No	No	No

As nouns, water moccasin and cottonmouth moccasin are synonyms defined as:

water moccasin and cottonmouth moccasin: venomous semiaquatic snake of swamps in southern United States

Other synonyms of water moccasin include Agkistrodon piscivorus, cottonmouth.

water moccasin (noun)	cottonmouth moccasin (noun)
venomous semiaquatic snake of swamps in southern United States	venomous semiaquatic snake of swamps in southern United States
any of numerous North American water snakes inhabiting fresh waters