Cut approximately half a medium watermelon into 1-inch cubes, removing all seeds. For optimal results, place cubes on a baking sheet lined with parchment paper and freeze for 1-2 hours.
Run a lime wedge around the rims of your serving glasses, then dip in salt or tajin seasoning. Chill glasses in the freezer for 10 minutes before serving.
In a high-powered blender, combine frozen watermelon chunks, tequila, triple sec, fresh lime juice, and agave nectar. Add ice gradually while blending until you achieve your desired consistency.
Taste your mixture and adjust as needed. If too sweet, add more lime juice; if too tart, incorporate additional agave nectar.
Pour the Frozen Watermelon Margarita into your prepared glasses. Garnish with small watermelon wedges and lime slices.