Combine 1 cup (236ml) of water with the granulated sugar in a small saucepan. Cook over medium-low heat, stirring frequently, until sugar is completely dissolved. Remove from heat and set aside.

Place watermelon in a blender and puree until completely smooth and pureed.

Pour watermelon through a fine mesh strainer into a large pitcher. Use a spatula to help nudge the juices through. Discard the pulp.

Add simple syrup, lemon juice and remaining 5 cups (1.18L) of water to the pitcher and stir well. Chill in the refrigerator until cold (stir before serving) or pour over ice to serve immediately. Garnish with additional watermelon, lemon slices, or mint sprigs. Note that the watermelon will settle as the lemonade sits and you should always stir well before pouring.