Basically what happens, if the cell phone has water in it, and there is still electricity flowing through it, it will short circuit different components within the phone. Once this happens, the phone is almost always never going to work again. The other issue that tends to occur is corrosion. Corrosion must be cleaned thoroughly to prevent components from short circuiting after the battery has been reinstalled.
Above is a picture of an iPhone 4 that i tried to revive. I cleaned the phone, piece by piece, and let it set for 24 hours before reassembling. But in the end, the phone did not come back up. I would say the primary reason was because the phone's battery was not removed initially when the incident occurred, and it had too much time to self destruct before i could tear it apart, and begin cleaning.
