Fill a large pot 3/4 full with water and bring to a boil. Add the pasta and cook until al dente (tender), 10 to 12 minutes, or according to the package directions. Drain the pasta thoroughly and rinse under cold water.
In a small saucepan, cover the asparagus with water. Bring to a boil and cook only until tender-crisp, about 3 to 5 minutes. Drain and rinse under cold water.
In a small bowl, add the salad dressing and dill. Whisk to mix evenly.
In a large bowl, add the pasta, asparagus, shrimp, tomatoes and scallions. Add the salad dressing mixture and toss to coat evenly. Cover and refrigerate until thoroughly chilled, about 1 hour.
To serve, divide the watercress among the plates. Top with dilled salad and serve.