narrow boat vs boat

narrow boat and boat both are nouns.

Word	Noun	Adjective	Verb	Adverb
narrow boat	Yes	No	No	No
boat	Yes	No	Yes	No

As nouns, boat is a hypernym of narrow boat; that is, boat is a word with a broader meaning than narrow boat:

narrow boat: a long boat that carries freight and is narrow enough to be used in canals
boat: a small vessel for travel on water

narrow boat (noun)	boat (noun)
a long boat that carries freight and is narrow enough to be used in canals	a small vessel for travel on water
	a dish (often boat-shaped) for serving gravy or sauce

narrow boat (verb)	boat (verb)
	ride in a boat on water