Stir broth and water together in a pot; bring to a boil.
Stir frozen tortellini into broth mixture and cook, occasionally stirring, until tortellini floats to the top and filling is hot, 6 to 8 minutes. Stir watercress and scallions into broth mixture and cook until watercress wilts, about 2 minutes. Add hot pepper sauce.
Sprinkle with Romano cheese and black pepper to serve.
