Stir together oats, water, half-and-half, apples, raisins, 1/4 cup maple syrup, pumpkin pie spice, vanilla, and salt in bowl of a 4- to 6-quart slow cooker. Cover and cook on low, 4 to 4 1/2 hours. (The cooker will switch to “warm” mode until you wake up.)
Spoon oatmeal into bowls. Drizzle with remaining syrup and, if using, pecans.