Pepper-Watermelon Margarita

In a small saucepan over medium heat, combine watermelon juice, serrano, salt and 1 cup of sugar. Stir until sugar dissolves, taste, and add more sugar if necessary. Remove the chile and let the syrup cool to room temperature. Meanwhile, rim glasses with salt. Combine the watermelon-chile syrup and remaining ingredients in a pitcher with ice cubes and stir to combine. Pour into ice-filled, salt-rimmed glasses and garnish.