WordNet records no synonym and no antonym for backwater. 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 body of water that was created by a flood or tide or by being held or forced back by a dam
“the bayous and backwaters are breeding grounds for mosquitos”
a place or condition in which no development or progress is occurring
“the country is an economic backwater”
Synonyms, antonyms and the broader and narrower relations for backwater come from Princeton University’s WordNet. Full sources and licences.