Salt the glass rims (optional): Rub a lime wedge around the rims of 4 rocks glasses. Fill a small plate with coarse salt. Dip each glass into the salt to coat the rim. Fill with ice.

Combine the tequila and half the jalapeño slices, if using. Set aside for 15 to 30 minutes to infuse (the longer you infuse, the spicier it will be). If you're not using the jalapeño, skip this step and proceed with the recipe.

In a blender, combine the watermelon with the lime juice and a handful of ice. Blend until smooth.

Pour the watermelon juice into the prepared glasses. Stir 2 tablespoons tequila into each glass. Taste and adjust, garnish with the lime slices, and serve.