Make the sauce: In a saucepan, whisk together water, sugar, cornstarch, and butter over medium-high heat. Bring to a boil, then lower the heat and stir in a few peach slices. Simmer until thick and smooth (about 5 minutes).
Cool completely: Remove from heat and let the sauce cool to room temperature.
Assemble the pie: Arrange the remaining peach slices in the graham cracker crust. Pour the cooled sauce evenly over the peaches, making sure they are fully coated.
Chill & serve: Refrigerate for at least 3 hours until firm. Slice and enjoy!