Firstly, you should quantify if your dog is actually drinking an excessive amount of water. In a 24 hour period, a dog should drink about 1 fluid ounce (or 30mL) per pound of body weight. Therefore, the recommended amount of water intake (in fluid ounces) equals your dog's weight (in pounds). For example, if your dog weighs 8 pounds, he/she should drink about a cup of water in a 1 hour period. This will be slightly increased if your dog gets a lot of physical activity or lives outdoors.
You can measure your dog's water intake the following way: in the morning, measure a specific amount, a little bit more than you think he/she will drink. 24 hours later, measure the remaining amount. If the amount of water your dog drank is significantly greater than it should be, then you should take your dog to a veterinarian.
Causes for mildly increased water consumption include food changes, increased ambient and body temperature, increased activity, urinary tract infection, and general illness.
Common causes for greatly increased water consumption include diabetes, urinary tract infection, kidney disease, steroid use, and other systemic diseases. With large increases in water consumption, you will also usually see increased urination. Please take note of urinary patterns to discuss with your vet. Greatly increased drinking and urination is ALWAYS a reason to see a vet.