I would withhold any additional food for 24-hours of the diarrhea, even if it's boiled chicken. I just lost my beloved beagle to kidney failure so I understand the worry, get him to a vet :(
Not knowing the cause, I would continue what you are doing until you can get her to the Vet. The fact she is drinking a lot of water may be an indication if a possible kidney issue going on, ESP due to her age. Your Vet can run a blood test that might reveal a condition that needs treatment, it will be good to have the dog get a good physical exam.
Good luck