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 2 batches if necessary. Strain through a sieve.
Cut watermelon in 1/2 crosswise. Hollow out one-half of the watermelon, forming a tureen. Seed flesh, and cut into 1/2-inch cubes. Pour soup into watermelon, and garnish with watermelon cubes. Reserve remaining 1/2 watermelon for another use.
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