Coat a large cast-iron skillet with cooking spray; place over high heat until hot. Add scallops; cook 2 minutes on each side or until browned. Remove scallops from skillet; set aside, and keep warm.
Coat skillet with cooking spray. Place over medium-high heat until hot. Add pepper strips and garlic; saute 2 minutes or until tender. Add vinaigrette and lemon juice, deglazing skillet by scraping particles that cling to bottom. Cook 1 minute. Return scallops to skillet, and cook until thoroughly heated.
Combine lettuce and watercress in a large bowl. Add scallop mixture to lettuce mixture; toss lightly. Sprinkle with 1/2 teaspoon pepper.