Using a cocktail shaker filled with ice, add the watermelon juice, grapefruit juice, lime juice, and tequila. Shake vigorously for about 20-30 seconds. Strain into a chilled glass filled with ice (or a single ice cube) that has been rimmed with grapefruit salt. Top with club soda or grapefruit soda. Garnish with a wedge of watermelon.
Preheat the oven to 200℉.
Place the salt and grapefruit zest on a baking sheet and mix well.
Cook for 60-70 minutes until the zest is completely dried out. Store in a sealed jar for up to a month.