If you're positive it's the belt squeaking than replace it. Are you 100% sure it's not a pulley with a bad bearing? A belt is a cheap and easy fix for a stock car so try that first.
Keep the old one as a spare in the car in case you break one on the road. Considering I can't pick up a spare belt from the parts store. Roush is the only one who sells my 136 inch long belt so I always keep one in the car and a 1/2" breaker bar.
Two old school ways you can stop a belt from squeeling. One, while the motor is running, gently place a bar of soap against the belt. Two, while the motor is off, put a dab of silicone sealant and the inside of the belt and then start and run your motor for a few to spread it evenly.
Both these methods work but is just a band aid till you get a new belt. Like Nasty said, I have an old belt in the trunk, just in case.
Water should only make the belt squeal while the belt is actually wet. Are you saying that the water made your belt slip, and caused wear, and now it squeals? I could see that happening. They make an aerosol for that available at any auto parts store. It's called belt dressing. Radiator Specialty M206 "Gunk" Stop Slip Belt Dressing 6 Oz.