Chop the watermelon into small chunks on a cutting board using a sharp knife, ensuring all seeds are removed.
Add the watermelon chunks, ground cumin, chili powder (if using), lime juice, and water into a Blender.
Secure the lid on the blender and blend on high speed until the mixture is completely smooth, with no chunks remaining, approximately 30 seconds.
Check the consistency; it should be thick and silky. If too thick, add a little more water and blend again for a few seconds.
Pour the smoothie into glasses, and serve immediately with optional garnishes like lime wedges or a sprinkle of cumin.