vessel vs fishing boat

vessel and fishing boat both are nouns.

Word	Noun	Adjective	Verb	Adverb
vessel	Yes	No	No	No
fishing boat	Yes	No	No	No

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

vessel: a craft designed for water transportation
fishing boat: a vessel for fishing; often has a well to keep the catch alive

vessel (noun)	fishing boat (noun)
a craft designed for water transportation	a vessel for fishing; often has a well to keep the catch alive
an object used as a container (especially for liquids)
a tube in which a body fluid circulates