Water damage is never good. It is best to immediately open the device and dry thoroughly when it has come into contact with water. Otherwise, the water can create many shorts and destroy components quickly or it can leave mineral deposits and create corrosion over time.
The water could have made its way into the display, the display connector, or a component on the logic board responsible for display, display output or backlight. It may be best to get it to a professional for proper diagnosis.