Cut fish across the grain into 2.5 cm/1 inch steaks, then cut each steak into 2.5 cm/1 inch squares. Combine remaining ingredients and spoon over fish cubes. Mix well, and leave to marinate for at least 2 hours, or overnight.
To grill fish, place fish cubes on metal skewers and grill over charcoal, turning skewers until fish is lightly browned on all sides.