In a blender, combine avocado, cilantro, 3/4 cup chicken stock, and serrano pepper. Add orange juice, tequila, salt, zest, and remaining 5 1/4 cups chicken stock. Blend until smooth, in two batches if necessary. Strain through a sieve.
Cut watermelon in half crosswise. Hollow out one half of the watermelon, forming a turreen. Seed flesh, and cut into 1/2-inch cubes. Pour soup into watermelon, and garnish with watermelon cubes. Reserve remaining half watermelon for another use.