Place fish 4 to 5 inches from hot coals; grill 10 minutes on each side or until fish flakes easily when tested with a fork. Remove from grill; set aside, and keep warm.
Lightly toast bread slices on both sides. Combine butter and lemon juice; beat until smooth. Spread over hot toast; cut toast in half. Serve grilled fish over toast, and garnish with lemon slices and watercress.