You should water your ginger about once a week. But while you only need to water once a week, ensure that you're watering thoroughly while doing so. You should add about 1" of water at a time, that way it can penetrate the soil and reach all the roots. Also, water your ginger early in the day, so it has plenty of time to dry out before night hits. Giving the soil the opportunity to dry out in a timely manner is necessary to avoid rot