captain has 10 distinct synonyms spread across 6 of its 8 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.
an officer holding a rank below a major but above a lieutenant
the naval officer in command of a military ship
a policeman in charge of a precinct
an officer who is licensed to command a merchant ship
a dining-room attendant who is in charge of the waiters and the seating of customers
Synonyms, antonyms and the broader and narrower relations for captain come from Princeton University’s WordNet. Full sources and licences.