You have to rotate and clean water bowls with every meal too. Read all about it...
Bacteria Alert: Why You Need to Keep Your Dirty Dog Bowls Clean
Dog water dishes are just as unsanitary as food bowls. When your dog drinks, they transfer germs from their mouths (and possible contamination from their food), and dog water at room temperature is an excellent habitat for bacteria. Could you be sure to give your pet fresh water at every feeding?