Add HALF the watermelon, HALF the cucumber, HALF the onion, HALF the bell pepper, and HALF the serrano pepper to a food processor and pulse until smooth.
Pour the puree into a large mixing bowl. Stir in the remaining, watermelon, cucumber, onion, bell pepper, serrano pepper, and 1/4 tsp salt. Stir in the vinegar and oil and season with salt and pepper to taste. Chill until ready to serve then ladle into bowls and garnish with the fresh chopped cilantro.