1. In a blender, combine the lemon juice, raspberries, blueberries and sugar. Cover and process until blended. Strain and discard seeds.
2. In a 2 1/2 quart pitcher, combine berry mixture and water. Pour into a freezer container. Cover and freeze for 8 hours or overnight.
3. Just before serving, remove from the freezer and let stand for 45 minutes or until slushy.