Step 1In 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 2Rinse 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.