For the espelette salt garnish: To prepare the glasses, combine the salt, piment d'espelette and sugar in a shallow dish. Wet the edges of 4 rocks glasses with a lime wedge, then dip into the mixture. Fill the glasses with ice.
For the margarita: Combine the watermelon juice, tequila, lime juice and triple sec in a cocktail shaker with ice. Shake vigorously, strain into the rimmed rocks glasses, garnish with lime wedges and serve immediately.