The only worry with opossums hanging around the food and water is that they can carry leptospirosis. My parents dog died from this, she was healthy and her organs shut down so fast. It was traumatic to see. The vets figured it was picked up off the grass from a raccoon or opossum.
Edit to add: she was vaccinated for it but it isn't 100% effective.
Edit to add: she was vaccinated for it but it isn't 100% effective.