Thaw fish, if frozen. Measure thickness of fish. Cut into 4 serving-sized pieces, if necessary. In a large skillet combine the 1 cup water and the 1/2 teaspoon salt.
For the bouquet garni, place 1/2 cup of the watercress, the 2 tablespoons tarragon, lemon slices, peppercorns, and bay leaves in the center of a double-thick, 9-inch-square piece of 100 percent cotton cheesecloth. Bring the corners of the cheesecloth together and tie with clean 100 percent cotton string. Place the bouquet garni in the skillet. Bring water to boiling; reduce heat. With a slotted spatula, gently lower the fish fillets into the water. Simmer, covered, for 4 to 6 minutes for each 1/2 inch of thickness or until fish just flakes when tested with a fork. Carefully remove the fish fillets with a slotted spatula to a platter. Cover fish and keep it warm while preparing sauce.
For sauce, in a blender container or food processor bowl combine sour cream, remaining 1/2 cup watercress, the 1 teaspoon tarragon, ground pepper, and salt. Cover and blend or process until smooth. Stir in chives. Spoon sauce onto plates and top with fish. Garnish with additional watercress and tarragon, if desired. Makes 4 servings.