Thread shrimp onto 12-inch skewers; sprinkle with 1/4 teaspoon salt and 1/4 teaspoon pepper. Place skewers on a grill rack coated with cooking spray; grill 3 minutes on each side or until done. Cool completely; remove shrimp from skewers. Place shrimp and beans in a large bowl; toss gently to combine.
Combine oil and lemon juice, stirring with a whisk; stir in remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Drizzle dressing over shrimp mixture; toss gently to combine.
Arrange watercress on a large platter, and top with shrimp mixture. Sprinkle evenly with chives. Serve with lemon wedges, if desired.