What is the relationship betweenn dehydration and sunken eye?

Dehydration. Since the body is 80% water, dehydration can lead to shrinkage of all the major tissues, including around the eye. When one is dehydrated, the eye appears sunken because the surrounding tissues are shriveling up. Try to drink 5-8 glasses of water per day, if you are of normal weight and are in good health.
Can be the cause. Dehydration can make the eyes sunken, particularly if it is severe. It is pretty rare to be that bad outside of survival situations.