In a small dish, mix the salt and sugar. Rub the rims of 3-4 margarita glasses with a little lime zest and then invert into the sugar and salt mixture to coat the rims of the glasses. Then make sugar syrup with the lime juice, zest and sugar, heating them together in a sauce pan just until boiling. Cool quickly by pouring into another shallow bowl and put in the freezer.
When the syrup is cool strain out the zest and place in blender with melon cubes, tequila and ice cubes in a blender, and blend until the drink is slushy and all ice is ground fine. Pour the drink into the prepared glasses and garnish with a triangle of watermelon and a slice of lime.