Use a sharp knife and cutting board to dice the watermelon into roughly 1-inch cubes, then transfer them into a large mixing bowl.
Add the minced garlic and sherry vinegar to the watermelon cubes. Stir gently to distribute the flavors evenly. Let sit for about 5 minutes to allow the vinegar to marinate the fruit slightly.
Transfer the marinated watermelon mixture to a blender. Blend on high speed until completely smooth and no chunks remain, about 30 seconds. The mixture should be a vibrant, homogeneous pink liquid.
Pour the blended soup into a clean bowl or container, then season with salt and freshly ground pepper to taste. Stir well to incorporate the seasonings.
Cover the bowl with plastic wrap and chill the soup in the refrigerator for at least 1 hour. The cold enhances the refreshing quality and helps the flavors meld together.
Serve the chilled watermelon soup in bowls, optionally garnished with fresh herbs, small watermelon cubes, or a drizzle of extra vinegar for added brightness. The final dish should be cold, smooth, and vividly colored, with a refreshing aroma.