Combine finely-chopped vegetables-herbs in a bowl and store in the refrigerator. They will be added after soup base is blended. (1c watermelon, 1c tomatoes, 1 scallion, 2 Tbs cilantro, 2 Tbs basil, cucumber, peppers, onions)
Make base: Put 5 cups of watermelon, 2 cups of tomatoes, 1 shallot, garlic, lemon juice, 2 TBS cilantro, 2 TBS basil, salt, pepper and cayenne into a blender and puree until smooth.
In a large bowl, stir to combine chopped veggies-herbs with pureed soup base.
Cover and chill for at least 1 hour.
Lightly stir reserved, finely chopped vegetables-herbs into chilled soup.
Serve chilled.