Soak shell or wood chips in water to cover at least 30 minutes.
Place fillets in a large heavy-duty zip-top plastic bag. Stir together 1 1/4 cups water and next 3 ingredients; pour over fillets. Seal and chill 2 hours, turning plastic bag occasionally. Remove fish from bag, reserving marinade. Set aside.
Prepare charcoal fire in smoker; let burn 15 to 20 minutes. Drain chips, and place on hot coals. Place water pan in smoker; add reserved marinade. Add hot tap water to fill line. Coat rack with cooking spray; place in smoker.
Arrange fish on rack, skin side down; cover with smoker lid. Cook 1 hour or until fish flakes easily when tested with a fork. Sprinkle with salt.