1. Scoop watermelons into balls using various size melon ballers, and place watermelon in a large bowl.
2. Whisk together lemon juice and next 4 ingredients in a medium bowl until sugar dissolves. Pour lemon juice mixture over watermelon balls; gently stir to coat. Cover and chill 1 to 2 hours.
3. Gently toss watermelon balls. Sprinkle with chopped fresh mint. Serve immediately with a slotted spoon.