Peel and devein shrimp; set aside.
Place fillet on a lightly greased rack in a broiler pan. Brush fillet with olive oil, and sprinkle evenly with salt and pepper. Broil 5 inches from heat 10 to 13 minutes or until fish flakes with a fork. Remove from pan, and cool.
Break cooled fish into large pieces, and place in a large bowl. Add shrimp, crabmeat, and next 3 ingredients; toss gently to combine. Drizzle with Dill Vinaigrette; toss gently to coat. Cover and chill at least 1 hour. Arrange seafood mixture on watercress.