Bring a large pot of water up to a boil. Score a large X on the back of each tomato, then carefully drop into the boiling water for about 20 seconds. Remove, then immediately transfer to a bowl of ice water. You want to stop the cooking process as quickly as possible, as the tomatoes should still be raw.**
Once cooled, peel the skins, then slice in half crosswise and remove the seeds, reserving them in a bowl. Cut tomatoes into chunks, then add to a blender. Mix the bread cubes with the tomato juice/seeds, then allow them to soak while you prepare the other ingredients.
Add cucumber, watermelon, onion, garlic, sherry vinegar, salt, and olive oil to the blender.
Squeeze out excess liquid from the bread and add to the blender, then blend on high for several minutes until you reach a smooth, creamy texture.
Taste for seasoning, and adjust as needed. Chill the gazpacho for a minimum of 30 minutes so that all the flavors can meld together, and it gets nice and cold.
Make the garnish right before serving. Mix together watermelon, cucumber, serrano chile, salt, olive oil, and basil in a small bowl.
Pour the chilled gazpacho into a bowl, then top with a spoonful of the garnish. Drizzle with more olive oil, and serve.