Combine fajita seasoning, the 1 cup salsa and water. Pour into a large zip-type bag, then add the Chicken. Marinate in the refrigerator for a minimum of two hours. Heat grill for medium coals. Remove Chicken from bag and reserve marinade. Grill Chicken, basting with reserved marinade, for 5 to 7 minutes on each side or until Chicken is done. Place on buns and top with Cheese, Onion, avocado and salsa.