Arrange cubed watermelon on a baking sheet covered with parchment paper. Transfer to the freezer and wait until the watermelon is fully frozen, about 1-2 hours.
Place the cubed frozen watermelon in a blender with a bit of water and blend for about 30 seconds until it turns into a liquid (start with 1-2 tablespoons and use as much water as needed to blend).
Add mezcal, lime juice, and simple syrup (if using) to the blender and process until a slushie consistency is achieved. Taste, and add more lime juice, or simple syrup if you prefer a sweeter taste.
Add the kosher salt to a small plate. Rub a lime wedge around the rim of 2 cocktail glasses and dip the rims into the salt.
Pour the watermelon mezcal margarita into the glasses.
Garnish with mint, small watermelon wedges, and lime wedges or slices.