Combine the ingredients for the marinade in a large bowl. Cut the fish into large cubes and add to the marinade with the onion, mushrooms and courgette. Cover and refrigerate for 1 hour to marinate.
Brush the rack of a grill pan lightly with oil to prevent the kebabs from sticking. Thread four skewers with the chunks of fish, mushrooms, courgette and onion alternately. Brush with a little oil and grill under a preheated hot grill for about 10 minutes, turning at intervals. Garnish with watercress or flat leaf parsley.