shallow has 1 distinct synonym spread across 3 of its 6 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. Its one recorded opposite is deep.
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.
lacking physical depth; having little spatial extension downward or inward from an outer surface or backward or outward from a center
“shallow water”
not deep or strong; not affecting one deeply
“shallow breathing”
1 of the 6 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for shallow.
Synonyms, antonyms and the broader and narrower relations for shallow come from Princeton University’s WordNet. Full sources and licences.