seaplane vs flying boat

seaplane and flying boat both are nouns.

Word	Noun	Adjective	Verb	Adverb
seaplane	Yes	No	Yes	No
flying boat	Yes	No	No	No

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

seaplane: an airplane that can land on or take off from water
flying boat: a large seaplane that floats with its fuselage in the water rather than on pontoons

Other hyponyms of seaplane include floatplane, pontoon plane.

seaplane (noun)	flying boat (noun)
an airplane that can land on or take off from water	a large seaplane that floats with its fuselage in the water rather than on pontoons

seaplane (verb)	flying boat (verb)
glide on the water in a hydroplane