fin has 17 distinct synonyms spread across 5 of its 9 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.
one of a pair of decorations projecting above the rear fenders of an automobile
a stabilizer on a ship that resembles the fin of a fish
organ of locomotion and balance in fishes and some other aquatic animals
propel oneself through the water in a finning motion
show the fins above the water while swimming
“The sharks were finning near the surface”
Synonyms, antonyms and the broader and narrower relations for fin come from Princeton University’s WordNet. Full sources and licences.