small boat vs dinghy

small boat and dinghy both are nouns.

Word	Noun	Adjective	Verb	Adverb
small boat	Yes	No	No	No
dinghy	Yes	No	No	No

As nouns, dinghy is a hyponym of small boat; that is, dinghy is a word with a more specific, narrower meaning than small boat:

small boat: a boat that is small
dinghy: a small boat of shallow draft with cross thwarts for seats and rowlocks for oars with which it is propelled

Other hyponyms of small boat include canoe, cockleshell, coracle, dory, rowboat, gig, racing boat, skiff, yawl.

small boat (noun)	dinghy (noun)
a boat that is small	a small boat of shallow draft with cross thwarts for seats and rowlocks for oars with which it is propelled