Preheat oven to 375 degrees F. In a large bowl toss together peach slices, brown sugar, flour, and ginger. Add blueberries and the water; toss to combine. Spoon fruit mixture into a 2-quart square baking dish. Bake, uncovered, for 20 minutes.
Meanwhile, place gingersnaps in a heavy plastic bag. Seal bag; using the flat side of a meat mallet or a rolling pin, crush cookies until in 1/4- to 1/2-inch pieces. Transfer cookies to a medium bowl. Stir in rolled oats and, if desired, chopped pecans. Stir in butter until well mixed. Sprinkle over partially baked fruit mixture.
Bake for 15 to 20 minutes more or until fruit is bubbly and topping is lightly browned. Cool on a wire rack for 30 minutes. Serve warm. If desired, top with whipped topping.