boat bug vs water boatman

boat bug and water boatman both are nouns.

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

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

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

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