flatboat vs pontoon

flatboat and pontoon both are nouns.

Word	Noun	Adjective	Verb	Adverb
flatboat	Yes	No	No	No
pontoon	Yes	No	No	No

As nouns, pontoon is a hyponym of flatboat; that is, pontoon is a word with a more specific, narrower meaning than flatboat:

flatboat: a flatbottom boat for carrying heavy loads (especially on canals)
pontoon: (nautical) a floating structure (as a flat-bottomed boat) that serves as a dock or to support a bridge

Other hyponyms of flatboat include dredger, houseboat, scow, Norfolk wherry, wherry.

flatboat (noun)	pontoon (noun)
a flatbottom boat for carrying heavy loads (especially on canals)	a float supporting a seaplane
	(nautical) a floating structure (as a flat-bottomed boat) that serves as a dock or to support a bridge