Excess water consumption in dogs is usually a sign of illness. Some of the medical conditions that result to this condition are diabetes, liver diseases as well as kidney infections. Certain medications can also cause excess water consumption.
1 Additional Answer
If your dog is drinking more water than usual, he could have a virus. Watch for excessive drinking because it could be a sign for polyuria. This could be kidney or liver disease. You can find more information here: http://blogs.chron.com/animalqa/2008/06/