Combine reduced-fat mayonnaise, yogurt, 1 1/2 teaspoons lemon juice, dill, 1/8 teaspoon salt, and pepper in a large bowl, stirring well with a whisk. Add fennel and next 4 ingredients (through beans); toss well to combine.
Combine oil, watercress, remaining 1 1/2 teaspoons juice, and remaining 1/8 teaspoon salt in a large bowl; toss gently to coat. Divide watercress mixture evenly among 4 plates; top each serving with 1 1/2 cups shrimp mixture.
Tip: For a flavor variation, try arugula and basil in place of watercress and dill. You can also substitute chopped rotisserie chicken for shrimp.