water level has 4 distinct synonyms spread across 2 of its 4 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
underground surface below which the ground is wholly saturated with water
“spring rains had raised the water table”
a line corresponding to the surface of the water when the vessel is afloat on an even keel; often painted on the hull of a ship
a water gauge that shows the level by showing the surface of the water in a trough or U-shaped tube
Synonyms, antonyms and the broader and narrower relations for water level come from Princeton University’s WordNet. Full sources and licences.