Of course, a belt will stop squealing when you put a lubricant, like water, on it! Duh! If the belt goes true across the pulleys, look for components which are loading the belt. Remove the belt, turn the various pulleys with hand, or, wrapping the belt around each, and "feel" how easily they turn. To check the A/C pulley: with the belt on, and the engine running, turn the A/C switch on the dash OFF and on. Watch the engine. It it seems to load with the A/C on, the A/C compressor is causing the squealing. Replace the A/C compressor.