Whisk together the soy sauce, water, dark brown soft sugar and rosemary in a bowl, and pour 1/2 of the marinade into a resealable plastic bag. Add the pork chops, coat with the marinade, squeeze out excess air and seal the bag. Marinate in the refrigerator for at least 3 hours. Set the remaining marinade aside.
Preheat an outdoor barbecue for medium heat and lightly oil the cooking grate. Remove the pork chops from the marinade and shake off excess. Discard the remaining marinade.
Place pork chops on the barbecue and cook until the pork is no longer pink in the centre, brushing occasionally with the reserved marinade, 4 to 5 minutes on each side. An instant-read thermometer inserted into the centre should read 70 degrees C.