Combine sugar, lime juice and 1/4 cup water in a small saucepan. Stir over medium-high heat until sugar has dissolved, about 5 minutes. Remove from heat and add lime zest. Cover pan and set aside for 15 minutes. Strain into a small bowl, cover and refrigerate until thoroughly chilled, at least 2 hours. (Can be made up to 3 days in advance.)
Cut each melon in half, remove and discard seeds and scoop flesh with a sharp 1-inch melon baller. Place all melon balls in a large bowl, cover and refrigerate until 30 minutes before serving.
Pour lime syrup over melon balls. Cover and chill for at least 30 minutes before serving. Divide into 8 tumblers and garnish with mint, if desired.