Place the watermelon chunks into a blender and blend until completely smooth, about 30 seconds, until a vibrant red liquid forms.
Pour the blended watermelon through a fine strainer or juicer into a large mixing bowl to remove pulp and seeds, resulting in a clear, bright pink juice.
Add the lime juice, tequila, and simple syrup (if using) to the watermelon juice. Stir well with a spoon to combine.
Fill glasses with ice cubes and pour the watermelon mixture over the ice until the glasses are nearly full.
Garnish each glass with a slice of watermelon or lime. Serve immediately with a straw or stirrer, observing the icy, pink appearance of the cocktail.