WordNet records no synonym and no antonym for tidal wave. 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.
an overwhelming manifestation of some emotion or phenomenon
“a tidal wave of nausea”
a wave resulting from the periodic flow of the tides that is caused by the gravitational attraction of the moon and sun
Synonyms, antonyms and the broader and narrower relations for tidal wave come from Princeton University’s WordNet. Full sources and licences.