How to Make It
Prepare Key Lime-Mint Sugar: Process fresh mint, Key lime zest, and 1/4 cup granulated sugar in a mini food processor until sugar is finely ground, 10 seconds. Place in a bowl, and stir in additional 1/4 cup sugar. Spread mixture on a rimmed baking sheet; let stand at room temperature until dry, 2 hours. Return mixture to food processor, and pulse until finely ground, about 4 times. Store in an airtight container in the refrigerator up to 3 months.
Process cubed melon in a juicer or blender. Strain through a fine mesh strainer, discarding solids. (You should have 1 1/2 cups juice.) Pour into ice cube trays; freeze 4 hours or overnight.
Combine watermelon ice cubes, grenadine, and lime juice in a blender; pulse until slushy, about 15 times.
Rim 2 Collins glasses with 1 tsp. Key Lime-Mint Sugar each. Pour the mixture into prepared glasses. Top with watermelon wedges.