Place the cubed watermelon into the blender and blend on high until completely smooth, about 30 seconds. Check for a vibrant, uniform pink color with a slightly thick texture.
Add the sherry vinegar to the blender and pulse a few times to incorporate, then taste and adjust with salt, pepper, or more vinegar as desired. Transfer the pureed mixture to a large mixing bowl and chill in the refrigerator for at least 15 minutes.
Meanwhile, dice the cucumber into small pieces and chop the mint leaves finely. Prepare the tiny croutons if not already prepared.
Remove the chilled watermelon soup from the refrigerator. Stir briefly, then ladle into individual bowls. Top each with chopped cucumber, fresh mint, and a handful of tiny croutons for added texture and visual contrast.
Optionally, garnish with extra mint leaves or a drizzle of sherry vinegar. Serve immediately with a spoon, enjoying the smooth, chilled texture complemented by crunchy toppings.