Place the watermelon chunks into the blender, filling it about halfway.
Add the ice cubes on top of the watermelon in the blender.
Secure the lid on the blender and blend on high speed until the mixture is smooth and frosty, about 30-40 seconds.
Stop the blender and check the texture; if it’s too thick, you can add a splash of water or some more watermelon and blend again for a few seconds.
Pour the slushie into glasses, filling them close to the top.
Serve immediately with a straw or a garnish if desired, enjoying the vibrant pink color and icy texture.