Put the water, barley, oat groats, and optional salt in a large saucepan. Bring to a simmer over medium-high heat. Decrease the heat to medium-low and cook, stirring occasionally, until the water is absorbed and the oats are tender, 1–1½ hours. Stir in the milk and raisins and cook for 15 minutes longer. If the porridge is too thick, add more milk or water as desired.