water boatman vs boat bug

water boatman and boat bug both are nouns.

Word	Noun	Adjective	Verb	Adverb
water boatman	Yes	No	No	No
boat bug	Yes	No	No	No

As nouns, water boatman and boat bug are synonyms defined as:

water boatman and boat bug: carnivorous aquatic bug having paddle-like hind legs

water boatman (noun)	boat bug (noun)
carnivorous aquatic bug having paddle-like hind legs	carnivorous aquatic bug having paddle-like hind legs