Watermelon (sandia) is one of Mexico’s best-loved fruits, often served with a squeeze of lime and a sprinkle of chile powder to bring out the juicy sweetness, or crushed with water and lime to make a favorite agua fresca. This drink playfully echoes those tastes, with a hint of mild guajillo chiles, basil to cut the sweetness and enhance the tequila flavors, and, just for fun, a sliver of candied cactus as a garnish. Look for candied cactus, or viznaga, at well-stocked Mexican markets. If you can’t find it, substitute candied watermelon rind or a triangular sliver of fresh watermelon with the green rind left on.