Originally Answered: I have a 16 yr old cat with diarrhea, what might be causing this?
At 16, it could be a number of issues -- developing allergies to food (particularly corn, wheat and wheat glutens), parasites/worms that an aging immune system cannot adequately handle, developing IBD, more serious kidney/diabetes issues....
Drinking a lot of water often points to worm, kidney, and diabetes issues.
You need to get your cat and get her a senior blood work. Even if you had one not long ago, things can change quickly in a cat that age.
In the interim, you can add a little canned pumpkin (no spices) to your cat's food, and/or a pinch of Metamucil (psyllium husks). These non-grain fibers will help to firm up the stool a little at least. Reassess your food, and look for senior foods for sensitive systems, the first ingredients being meats (or water/broth if wet), with low/no grains, avoiding corn, wheat and wheat glutens.
My Rusty was diagnosed with severe IBD, and with only one kidney, it has been an issue. We control the condition AND maintain his health by good cat-food (Fancy Feast actually works well for him, Max Cat, Innova wet, and Nutro senior sensitive system dry), and by adding enzymes I get from Dr. Goodpet. The enzymes not only help with stool formation, but help with digestion and nutrient absorption. If your cat is diagnosed with IBD, it can take a while and some experimenting to find the right food formula that works, so don't give up. Rusty just turned 18, by the way, and I fully expect the little scamp will be with me a lot longer.
Other issues can be handled with treatments, diets, and care, but you have to know what they are before you can do that.
However, the first course of action is to assess your cat's diet, and a visit to the vet to determine any health issue. Take a fecal sample (no matter how sloppy), and at least two urine samples from different times of day.