In a small bowl, stir together the marinade ingredients. Remove 1/2 cup of marinade and reserve for basting the chicken later. Pour the rest of the marinade in a 13x9-inch pan. Add the chicken breasts and marinate at room temperature for about 30 minutes, turning once or twice.
Meanwhile, heat grill to medium-high. When grill is ready, lightly oil the rack.
Remove the chicken from the marinade and place the chicken on the rack. Close the grill, basting often with the reserved marinade until it is used up, and turning the breasts every 2 minutes. Continue to grill for at least 2 minutes after the marinade is used up, turning the breasts once, until just done through while remaining juicy. Total grilling time should be about 8 minutes. Transfer the chicken breasts to a cutting board and let them rest for 3 to 4 minutes.
Across the grain and at a slight angle, carve the chicken breasts into thin slices. Season to taste with salt and pepper and serve hot or warm.
*WEIGHT WATCHERS FREESTYLE SMARTPOINTS PER SERVING (4 servings): 4
*Serve with grilled pineapple and lime wedges, for squeezing (optional).
*If you are preparing this recipe as gluten free, just be sure to use brands of soy sauce and ketchup that are known to be GF.