In a small saucepan, make a simple syrup by bringing ½ cup water and sugar to a boil over high heat. Reduce to medium-low and simmer until sugar is dissolved, stirring frequently. Remove from heat and allow to cool.
Puree watermelon slices in a blender and until smooth. Strain through a fine mesh strainer to remove any seeds or pulp.
Add the 3 cups of watermelon puree, cooled simple syrup, lemon juice, and 3 cups of water to a pitcher. Stir gently to combine.
Chill in the refrigerator until ready to serve. To serve, pour over glass of ice cubes and garnish with a wedge of watermelon, if desired.
Notes
If the beverage sits for a while it will separate a bit, so give it a quick stir before serving.
For a lighter drink replace the sugar with your favorite sugar substitute.
Turn this into a tasty watermelon lemonade cocktail by adding a splash of vodka or rum.
Use a lemon squeezer to get the most juice out of your lemons.