To create a salt rim, pour salt onto a plate until it reaches a depth of 1/4 inch. Rub a watermelon slice on the rim of a margarita glass or dip the glass in water. Dip the wet rim of the margarita glass in the salt.
Prepare the margarita either as a frozen cocktail or on the rocks. For a frozen margarita, place the sour mix, watermelon syrup, tequila, and 1 cup of ice into a blender and puree. For a margarita on the rocks, fill the salt-rimmed margarita glass with ice. In a separate glass, stir together the sour mix, watermelon syrup, and tequila.
Pour the mixture into the salt-rimmed margarita glass and garnish with a watermelon slice.