Preheat the oven to 350 degrees. To make the filling, add the peaches and water to a small saucepan and bring to a simmer over medium heat. Cover and simmer five minutes, or until the peaches are tender. While the peaches are simmering, whisk together the sugar and corn starch. After the peaches have softened, stir in the sugar mixture and continue to simmer 2-3 minutes, or until slightly thickened. Remove from heat and let cool while you make the cake.