WordNet records no synonym and no antonym for breaststroke. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
a swimming stroke; the arms are extended together in front of the head and swept back on either side accompanied by a frog kick
swim with the face down and extend the arms forward and outward while kicking with the leg
Synonyms, antonyms and the broader and narrower relations for breaststroke come from Princeton University’s WordNet. Full sources and licences.