Rim the glasses. Add some Tajín to a shallow small plate. Run a lime wedge around the rims of your serving glasses, then dip the rims lightly in the Tajín. Set the glasses aside until ready to use.
Blend. Add the frozen watermelon chunks, lime juice and water to a blender. Cover and blend until smooth, adding in more water as needed to get the blender going and achieve a thick slushie consistency.
Serve. Pour the mixture into your serving glasses, top each with a slice of lime for garnish if desired, then serve and enjoy!
Notes
Tips on freezing watermelon: In order to prevent the diced watermelon from freezing in one big clump (or sticking to your bowl), I recommend dicing the fresh watermelon into 1-inch cubes. Then place the cubes on in an even layer on a parchment-lined baking sheet, and freeze for at least 2 hours.