In a blender blend coriander and serrano with 1/4 cup orange juice until mixture forms a smooth paste. Add remaining 3/4 cup orange juice, zest, Tequila, broth, and avocados and blend until smooth. Pour mixture through a coarse sieve into a large bowl and add white pepper and salt to taste. Chill soup, covered, 2 hours, or until cold.
Serve soup topped with watermelon