Place the watermelon chunks into the blender, ensuring they are evenly distributed for smooth blending.
Add the lime juice and simple syrup to the blender.
Blend the mixture on high speed until the watermelon is broken down into a bright pink juice with no large chunks, about 30-45 seconds.
Pour in the tequila and add a handful of ice cubes to the blender; blend again until smooth and slushy, approximately 20 seconds. The mixture should be cold, slightly thickened, and homogenous.
Pour the liquid into glasses filled with additional ice if desired, and garnish with a small wedge of watermelon or lime slice, if preferred.