Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am sorry to hear that your girl is drinking (and likely urinating) a significant amount more then usual.
Is she eating more or less then usual?
Is she losing or gaining weight?
A normal dog will take in 1 to 2 ounces of fluids per pound of body weight in a 24 hour period. So an 8 pound dog will normally take in 8 to 16 ounces in a 24 hour day (1 to 2 eight ounce cups). They drink more when eating dry food and less with canned because canned is primarily water. I would measure the amount of water she drinks in a 24 hour period to see if she truly is drinking too much. Put out a measured amount and then keep track of any added and subtract what is left at the end of a 24 hour period.
If she is eating dry food and it is warm where you are and she is drinking more then 2 ounces per pound of body weight over a 24 hour period or more then 1 ounce per pound of body weight if she is eating canned food and the weather has been cool then she should be seen by a veterinarian and have some testing done.
It sounds like she has seen two veterinarians, but you don't mention whether any testing has been done.
I would be concerned about internal organ disease (especially kidney disease, but liver disease is possible too) or an infection, especially in his urinary tract (if she is not spayed a uterine infection would be a concern) as well as ketoacidotic diabetes or some types of cancer that cause increased water consumption and thus increased urination.
Cushing's disease (hyperadrenocorticism) is another possible cause of increased water consumption as are other endocrine abnormalities like hyporthyroidism or less commonly pituitary gland abnormalities in growth hormone secretion or antidiuretic hormone secretion.
I highly recommend that your girl see her veterinarian for an examination, complete blood count and biochemistry profile as well as a urinalysis and maybe abdominal radiographs or an ultrasound to diagnose her condition.
Further testing will depend upon the results of those tests.
I understand that you may be very frustrated with a lack of answers so far, but she may be a complicated case that may take more intricate or unusual testing to find the cause of her symptoms, and then develop a plan to treat her.
In the meantime do not restrict her water, let her have as much as she wants. Dogs that have a significant drive to drink can become dangerously dehydrated and end up with significant electrolyte imbalances if water is restricted.
Please let me know if you have any further questions.