In a small pot, bring the water, milk, and salt to a boil. Slowly stir the grits into the boiling mixture. Stir continuously and thoroughly until grits are well mixed. Let the pot return to a boil, cover with a lid, lower the temperature, and cook for approximately 30 minutes, stirring occasionally. Add more water if necessary. Grits are done when they have the consistency of smooth cream of wheat. Stir in the butter and serve.