swimmer vs aquanaut

swimmer and aquanaut both are nouns.

Word	Noun	Adjective	Verb	Adverb
swimmer	Yes	No	No	No
aquanaut	Yes	No	No	No

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

swimmer: a person who travels through the water by swimming
aquanaut: an underwater swimmer equipped with a face mask and foot fins and either a snorkel or an air cylinder

Other hyponyms of swimmer include floater, skin-diver, surfboarder, surfer.

swimmer (noun)	aquanaut (noun)
a trained athlete who participates in swimming meets	an underwater swimmer equipped with a face mask and foot fins and either a snorkel or an air cylinder
a person who travels through the water by swimming	a skilled worker who can live in underwater installations and participate in scientific research