Combine the tequila and half of 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, and blend until smooth.
Salt the rim of 4 glasses, if desired, and fill each with ice. Pour the watermelon lime juice into the glasses and stir 2 tablespoons of tequila into each glass. Garnish with the lime rounds.