Thank you for your question. There are many reasons why your pet could be exhibiting these symptoms.
In older pets we have to consider that there may be significant dental disease that could be contributing to the film in the mouth and older pets are also at increased risk of metabolic diseases such as kidney disease, diabetes, liver disease etc.
The increased drinking is always a reason to have your pet examined by a veterinarian and have blood work completed.
I diagnose and treat so many pets with increased drinking and urinating with kidney disease, diabetes, Cushing's disease etc. And I always encourage owners that have pets over the age of 7 to have routine blood work completed. In addition, we could have multiple things going on at once.
Do you currently have a veterinarian close to you that you trust?
It is always best to have this evaluated asap. Without blood work and an exam it is difficult to determine the cause.
I really hope that helps, but please do not hesitate to contact me if you need clarification or if you have further questions.
I am more than happy to help you out in any way that I can. Plus any additional information that you can provide about Sushi would be helpful.
Hope to hear from you soon.