Could the water my Maltese drinks have anything to do with staining?
Yes. High mineral content in your water could cause almost the entire face to stain. One suggestion would be to train your Maltese to drink from a water bottle which will help keep the face dry. Another solution would be to purchase purified bottled water.
After an extensive 2 year test on 15 different Maltese of different ages from different bloodlines, the results showed that tap water vs bottled water made absolutely no difference in the amount of tearstain although there was a noticeable improvement in the amount of face stain using the bottled water vs both tap and well water.