1. Heat oven to 450 degrees. Spray two 15 x 11-inch baking pans with nonstick cooking spray.
2. Place zucchini, squash, carrots and red peppers in a large bowl. Toss with 2 tablespoons of the dressing. Spoon vegetables evenly into prepared baking pans and season with pepper and salt. Roast at 450 degrees for 30 minutes, turning once after 15 minutes.
3. While vegetables are roasting, place shrimp in a resealable plastic food-storage bag with 2 tablespoons of the dressing. Shake to coat. Marinate in refrigerator 15 minutes.
4. Grill or broil shrimp for 2 minutes per side. Set aside.
5. Evenly distribute watercress among 6 large salad plates and top with roasted vegetables. Drizzle each with 1 tablespoon of remaining dressing. Scatter cooked shrimp and feta over each salad. Serve immediately.