What is a good substitute for milk in oatmeal?

When making oatmeal, I use plain tap water.

If you use the small, one serving bags, carefully cut or tear the bag at the dotted line, empty the contents into a bowl and then fill the empty paper envelope with tap water.

Pour the water into the oatmeal and microwave on high for 90 seconds, or follow the directions for heating over a burner.