Electronic gurus swear by it. I don't drive my RC's in water because it just does other damage over time. Bearings, rust, etc. So I have only used it a couple times. It worked on the WPL stuff I used it on. Truck went underwater just fine.
Buy some conformal coating. Pull the servo apart and coat the PCB with it. Presto. Waterproof. If water gets in it, it can still mess things up though. But it will be better than nothing.