WordNet records no synonym and no antonym for shark. 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.
any of numerous elongate mostly marine carnivorous fishes with heterocercal caudal fins and tough skin covered with small toothlike scales
a person who is unusually skilled in certain ways
“a card shark”
Synonyms, antonyms and the broader and narrower relations for shark come from Princeton University’s WordNet. Full sources and licences.