pontoon vs flatboat

pontoon and flatboat both are nouns.

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

As nouns, flatboat is a hypernym of pontoon; that is, flatboat is a word with a broader meaning than pontoon:

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

Other hypernyms of pontoon include barge, hoy, lighter.

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