Dissolve salt in water.
Stir together shallots and next 8 ingredients in a large bowl; stir in salt water. Set aside 2 cups mixture to serve with fish.
Place 1 grouper fillet, skin side down, on a sheet of plastic wrap in a 15- x 10-inch jellyroll pan. Spoon remaining sauce over fillet; top with remaining grouper fillet, flesh side down. Cover and chill 3 to 6 hours, turning occasionally.
Place grouper fillets, skin side down, on grill rack. Grill, covered with grill lid, over medium-high heat (350° to 400°) 40 minutes or until fish flakes with a fork. Serve with reserved sauce.