Per CC-CEDICT: 'to turn traitor; to defect.' Originally
江湖/triad slang for switching gang allegiance. Modern senses: in fan circles or politics, the 'defection' meaning dominates (a fan who
反水 to a rival camp); in e-commerce affiliate marketing,
反水 means cashback paid back to the buyer when they purchase via your link — a riff on 'water flowing back.'