flying boat vs hydroplane

flying boat and hydroplane both are nouns.

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

As nouns, hydroplane is a hypernym of flying boat; that is, hydroplane is a word with a broader meaning than flying boat:

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

Other hypernyms of flying boat include seaplane.

flying boat (noun)	hydroplane (noun)
a large seaplane that floats with its fuselage in the water rather than on pontoons	an airplane that can land on or take off from water
	a speedboat that is equipped with winglike structures that lift it so that it skims the water at high speeds

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