Prepare watermelon juice by cutting up the watermelon and removing the rind. Cut watermelon into large chunks and place into a blender. Blend until smooth. Optionally, you can strain this juice to make it clearer.

In a cocktail shaker, add ice, prepared watermelon juice, tequila, and St. Germain if using.

Shake until cold.

(Optional) To rim the glass, take a wedge of lime and run it around the edge of the glass. Pour salt or Tajin seasoning onto a plate, then dip the wetted edge of the glass onto the seasoning on the plate..

Strain over ice in a rocks glass. Garnish with a lime wheel and herbs if using.