Place the watermelon cubes in a blender and process until smooth, creating about 2 cups of fresh watermelon juice with a vibrant pink color.
Use a muddler to gently crush the basil and mint leaves in a small bowl or directly in the shaker, releasing their bright aromatic oils and color.
Pour the watermelon juice into a cocktail shaker and add the muddled herbs, tequila, lime juice, and simple syrup.
Add a handful of ice cubes to the shaker, then secure the lid and shake vigorously for about 15 seconds until well-chilled and slightly frothy.
Prepare the glass by briefly rubbing a lime wedge around the rim and then dipping it into a plate of salt to create a salted rim.
Strain the cocktail into the prepared glass, pouring slowly to keep the herbs and pulp from clouding the drink.
Garnish with extra basil and mint leaves for aroma and visual appeal, and serve immediately while chilled.
