swell has 29 distinct synonyms spread across 7 of its 11 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.
increase in size, magnitude, number, or intensity
“The music swelled to a crescendo”
the undulating movement of the surface of the open sea
a rounded elevation (especially one on an ocean floor)
Synonyms, antonyms and the broader and narrower relations for swell come from Princeton University’s WordNet. Full sources and licences.
a crescendo followed by a decrescendo