Place cubed watermelon into a blender. Cover, and puree until smooth. Strain through a fine mesh sieve. (I use about 3/4 of a seedless watermelon to get 4 cups of juice.)

Make simple syrup by bringing sugar and 1/2 cup water to a boil in a saucepan over medium-high heat until sugar dissolves, about 5 minutes. Remove from heat. If you want, the syrup can be made ahead and refrigerated in a glass jar for up to 1 month.

Stir together 4 cups of water,lemon juice, watermelon juice, and simple syrup. Optionally, stir in 1 cup fresh or frozen blueberries. Serve chilled.