First, cut your watermelon into chunks, place it on wax paper then place it in the freezer until frozen. If it's not frozen, your margarita will be a little gritty and you really don't want that.
Run a lime wedge along the rim of each serving glass and dip the glass into the course salt. Works best if you add the course salt to a shallow plate and dip the glass right in.
In a blender, combine your frozen watermelon, ice, lime juice, agave nectar, tequila, and triple sec. Pulse until everything is combined and slushy.
Add the chopped herbs (if desired) pulse a few more times and serve!
Notes
Herbs are optional. You can make this drink ahead of time, but if you do it this way I recommend placing it into the fridge for no more than 30 minutes. *Nutrition facts are automated.