soft water has no recorded synonym. 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. Its one recorded opposite is hard water.
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.
water that is not hard (does not contain mineral salts that interfere with the formation of lather with soap)
Synonyms, antonyms and the broader and narrower relations for soft water come from Princeton University’s WordNet. Full sources and licences.