Step 1 In a blender, puree the watercress, cilantro and broth. Set the mixture aside for 10 minutes to steep, then strain, discarding the solids and reserving the liquid.
Step 2 Rinse the blender, and puree the avocados, sour cream, yogurt, cumin, coriander and cayenne until smooth.
Step 3Pour the puree into a bowl set over an ice bath and whisk in the reserved watercress liquid, 1 cup at a time. Continue to stir until well-chilled. Season with salt. This makes about 6 1/2 cups of soup.
Step 4Pour the soup into bowls, and divide the shrimp among the bowls. Garnish each with a sprig of cilantro and serve immediately.