To make the watermelon juice, blend watermelon cubes in a blender until smooth. Strain out the pulp using a fine mesh strainer.
Run a lemon wedge around the edges of your glasses and dip it into salt.
Fill a cocktail shaker with ice. Pour in the tequila, lime juice, agave nectar and mint sprigs. Shake for 30 seconds. Strain it into your rimmed glasses.
Pour watermelon juice over the glasses and add extra ice cubes in the margaritas if you like.