Stir together the water, sugar, cornstarch, and butter in a small saucepan over medium-high heat. When the mixture has come to a boil, add a few slices of the peaches, then lower heat to medium-low and simmer for 5 minutes until thick and smooth. Remove from the heat, and allow to cool completely.
Place the remaining peach slices into the pie crust, and spread the sauce all over them, covering completely. Refrigerate for at least 3 hours, or until firm before serving.