river boat vs showboat

river boat and showboat both are nouns.

Word	Noun	Adjective	Verb	Adverb
river boat	Yes	No	No	No
showboat	Yes	No	No	No

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

river boat: a boat used on rivers or to ply a river
showboat: a river steamboat on which theatrical performances could be given (especially on the Mississippi River)

Other hyponyms of river boat include keelboat.

river boat (noun)	showboat (noun)
a boat used on rivers or to ply a river	a river steamboat on which theatrical performances could be given (especially on the Mississippi River)