Rinse the fish fillets and place them in a shallow glass baking dish. Drizzle olive oil over the fish and season with garlic and rosemary. Rub the seasonings into the fish. Cover and refrigerate overnight.
Dissolve the curing salt in the water and pour into the dish with the fish. Let it marinate for 15 minutes per half inch of thickness, taking care not to marinate for too long or the fish will become laden with liquid.
Meanwhile, prepare your BBQ smoker for a four hour slow burn using charcoal. The temperature should be at 150 degrees F (65 degrees C) before you get started.
Remove the fish from the brine and discard leftover liquid. Place each piece of fish onto a small piece of aluminum foil - just big enough to hold the fillet, and season with pepper to taste. Place them on the rack in the smoker. Sprinkle a handful of the soaked wood chips over the coals or place them into in a heat box.
Cover and allow fish to smoke for 2 hours, adding more wood chips as needed.
Increase the heat in the BBQ smoker (add more charcoal as required) until it hits 200 degrees F (95 degrees C) and let the fish smoke until the internal temperature of the fillets reaches 165 degrees F (72 degrees C). Remove the fish from the BBQ smoker and allow it to rest properly for 20 minutes before serving.