Preheat broiler. Mince enough watercress to make 2 tablespoons. Reserve several sprigs for garnish. Mix minced watercress with lemon juice, yogurt, pepper and salt.
Brush each side of fish with oil, if desired, and sprinkle with dash of paprika. Broil about 3 inches from heat source 8 minutes; turn and broil 8 to 10 minutes more until fish flakes easily with a fork. Garnish with watercress sprigs and a small amount of the lemon-watercress sauce.