It's possible to get dark circles under eyes because of dehydration,but that's not the only reason.Dehydration, or excessive water loss from the body, can cause darkness under the eyes. Drinking enough fluids can help prevent dehydration.
The other reason might be poor blood circulation,which to the head or the skin is impaired, discoloration under the eyes can occur. With regular exercise and/or lying with down with the feet elevated, circulation can be improved, and the dark circles may diminish.
And sleep disorders, such as insomnia or waking up multiple times during the night, can cause under eye discoloration or exacerbate already-present dark eye circles. When the body is not well rested, it is under stress. Adequate sleep gives the body, including the skin, the chance to repair and restore
On top of that,I hope you can have a totally under standing of your own situation,and get rid of it as soon as possible.