Measure and pour the tequila, triple sec, lime juice, and simple syrup (if using) into a blender jar. Add the cubed watermelon and ice and blend until smooth.
Serve:
Pour the frozen margarita into four glasses, or keep extras in the freezer for refills.
Notes
If using frozen watermelon reduce the amount of ice in half.
Make it a single cocktail by blending ½ cup of watermelon, 1 ½ ounces of Tequila, 1 ounce of Triple Sec, 1 ounce of lime juice, ½ ounce of simple syrup, and a handful of ice.