In a saucepan, bring water and sugar to the boil. Cook and stir until sugar is dissolved, about 5 minutes. Cool. Add the lemon juice and grated zest.
Pour into the bowl of an ice cream freezer; freeze according to manufacturer's directions. Allow to ripen in ice cream freezer or firm up in the refrigerator freezer for 2-4 hours before serving. Remove from the freezer 10 minutes before serving.