water boatman vs water bug

water boatman and water bug both are nouns.

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

As nouns, water bug is a hypernym of water boatman; that is, water bug is a word with a broader meaning than water boatman:

water boatman: carnivorous aquatic bug having paddle-like hind legs
water bug: a true bug: large aquatic bug adapted to living in or on the surface of water

water boatman (noun)	water bug (noun)
carnivorous aquatic bug having paddle-like hind legs	a true bug: large aquatic bug adapted to living in or on the surface of water
	small light-brown cockroach brought to United States from Europe; a common household pest