Rinse steaks thoroughly with cold water; drain and pat dry. Place steaks in a large shallow container. Combine white wine, lemon juice, soy sauce, Worcestershire sauce, and garlic; pour over steaks. Cover and marinate at least 4 hours, turning steaks twice.
Remove steaks from marinade; discard marinade. Lightly coat one side of each steak with pepper. Place steaks in a wire grilling basket. Grill over medium coals 18 minutes on each side or until fish flakes easily when tested with a fork.
Transfer steaks to a serving platter; garnish with watercress, if desired. Serve immediately with Tangy Mustard Sauce.