Place the tomatoes, watermelon, cucumber, bell pepper, shallot, basil and mint in a large bowl. Season with 1 1/2 teaspoons salt, and pepper to taste. Stir in the olive oil and vinegar.
Puree the tomato mixture in a blender, in batches if necessary. Strain through a fine-mesh sieve into a bowl, pressing to extract any liquid; discard the solids and season with salt and pepper. Cover and refrigerate until chilled, about 1 hour.
To serve, top the soup with the small herb leaves and a drizzle of olive oil.