Peel and seed the tomatoes by scoring a small x in the bottom of each tomato. Drop tomatoes into a large pot of boiling water and blanch for 30 seconds. Remove to an ice bath for several minutes and when cool enough to handle, remove the skin from the tomatoes. Roughly chop. In a food processor or blender, combine the chopped tomatoes, cucumbers, shallots, red bell peppers, cilantro, vinegar, olive oil, salt, pepper and optional red pepper flakes. Pulse until liquified. Add the watermelon and blend well. Here you may need to work in batches.Refrigerate for 4 hours and serve in chilled cups topped with cilantro leaves and a drizzle of olive oil.