In older dogs (and although she's only 9, this is considered older), the first thing I think of when I hear that they're drinking a lot of water is that they could be diabetic. Polyuria (increased urination) and polydipsia (increased drinking) are two of the hallmark signs for diabetes.
The symptoms you describe: polydipsia (excessive water drinking) of course can be due to a number of different causes. The most common causes of PU/PD in dogs include kidney disease, Cushing's disease (excessive blood cortisol levels), and diabetes mellitus (excessive blood glucose levels). In an 15 year old dog, any one of these disease processes could be occurring.
Getting her to your vet for a thorough physical examination and blood work is the best thing you can do. Try to collect a urine sample as well so that your vet can evaluate kidney function and look for signs of urinary tract infection. Since your dog is a female, allow your dog to begin urinating and when you are sure she is peeing you can slip a shallow bowl or soup ladle into the urine stream to catch your sample. Put the urine in a clean, dry container and refrigerate until your vet appointment. It's best if the sample is less than 24 hours old.
Other possible causes for PU/PD include inflammation of the prostate (rare in a neutered male), liver problems, or diabetes insipidus (a disorder of water metabolism in which part of the brain does not secrete the hormone which tells the kidneys to re-absorb water, or the kidneys do not respond to the hormone). In addition, there is a condition called psychogenic polydipsia which means the excessive water drinking is all in his head. If he exhibits other symptoms like vomiting, diarrhea or lethargy, other causes should also be investigated. Blood work often helps narrow the field.
I hope this helps!!
I apologize, I was answering another question at the same time...I did make the comment that she was 9 in the first paragraph.
The yellow stool can be due to excess bilirubin in the feces, which could indicate a problem with her liver. Having the vet run bloodwork willl tell you whether she's got diabetes and whether her organs are in good working condition.