argonaut has 3 distinct synonyms spread across 1 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
someone engaged in a dangerous but potentially rewarding adventure
(Greek mythology) one of the heroes who sailed with Jason in search of the Golden Fleece
cephalopod mollusk of warm seas whose females have delicate papery spiral shells
Synonyms, antonyms and the broader and narrower relations for argonaut come from Princeton University’s WordNet. Full sources and licences.