Preheat grill to MEDIUM. Lightly oil grill grate. Rub garlic into chicken. Grill over indirect heat for 15 to 20 minutes, turning occasionally, until juices run clear.
Watch carefully to prevent burning! Meanwhile, prepare salsa. In a medium bowl, combine remaining ingredients (watermelon through cilantro) and toss well to combine.
Top chicken with watermelon salsa and enjoy!