How to Make It
Combine sugar and water in a saucepan. Cook over high, whisking occasionally, until sugar dissolves, 4 to 5 minutes. Remove from heat, and add mint. Let stand, uncovered, 15 minutes. Strain into a bowl and discard mint. Chill syrup until ready to use.
Process watermelon in a blender until smooth. Strain into a large pitcher (you should have about 4 cups. If you have more, reserve for another batch); discard solids.
Stir together water, lime juice, 4 cups watermelon juice, and ½ cup syrup in a pitcher. (Reserve remaining Mint Simple Syrup for another use). Serve over ice, and garnish with mint sprigs.