Heat juice in small pot on high until reduced by half. Add apple jelly and whisk until combined. Add lime juice, oregano and salt. Pour half into separate bowl for salad. Set both aside.
Preheat grill to medium-high heat. Place chicken on grill and cook 4 minutes and flip. Baste tops of chicken with watermelon glaze and cook 4 minutes. Flip and baste tops with glaze. Cook another 2-5 minutes, or until chicken is cooked through Remove from grill and top with feta.
Serve with arugula topped with watermelon and drizzled with unused glaze from second bowl.