This soup is sweet and savory; it's the perfect cold meal for a summer evening. Make sure to serve it well chilled.
Put 5 ½ cups of the watermelon, 2 ½ cups of the tomatoes, shallot, oil, vinegar, 2 tablespoons of the cilantro, salt, and pepper into a blender and purée until smooth. Transfer the purée to a fine-meshed strainer set over a large bowl and strain the purée, pressing as much through as possible. Discard the solids. Cover and chill for at least 1 hour.
Finely chop the remaining watermelon, tomatoes, and cilantro and toss together in a bowl. Pour the chilled soup into bowls or glasses and top with the watermelon-tomato mixture.