Boil water and sugar, stirring to dissolve the sugar, then remove from the heat and add lime juice, tequila, triple sec and salt. Let cool, then stir in grated lime zest and pour into a small glass baking dish; cover and freeze for about 4 hours before scraping with a fork. For the full effect, dip the rims of the serving bowls in equal parts sugar and kosher salt, and a pinch of lime zest. Garnish with lime slices.