A stainless steel bowl is best. Stainless steel is bacteriostatic, it doesn't chip or crack and is easy to keep clean. Some plastics are known to discolor faces. Plastic absorbs odors. Food or water left in plastic containers may have an unpleasant odor, undetected by humans but quiet obvious to dogs who have a much more developed sense of smell.
Some folks like to add a little cider vinegar to the water or even lemon juice.
We found that our dogs didn't seem to drink as well. Adding about 100-200 mg. Some vets say it works, others say it doesn't. A naturopathic vet recommended that I try a zinc supplement for face stain. If tear ducts are blocked, often tears will spill over and possibly stain the face.