Simmer the Oats: In a small saucepan over medium heat, bring the milk, water, and sea salt to a gentle simmer. Stir in the rolled oats, finely diced apple half, ground cinnamon, nutmeg, and maple syrup. Reduce heat to medium-low and simmer uncovered for 8 to 10 minutes, stirring occasionally, until the oats are thick and creamy. Remove from heat and stir in the vanilla extract. Cover and let stand for 2 minutes.