Place the watermelon, sugar, lemon juice, and 1 cup of water in a blender. Blend until smooth. Place a fine mesh sieve over a pitcher and pour the mixture through the sieve. Add the remaining water to the pitcher and stir until combined. Chill and serve over ice.
Notes
Adjust the recipe to suit your taste. You can easily add more sugar or lemon juice to make it more sweet or tart.