In a small saucepan over medium heat, combine the water and sugar substitute, and boil until the sugar substitute dissolves, stirring constantly. Remove from heat and allow to cool. Stir in the lemon juice. Transfer to ice cream maker and churn according to manufacturer instructions. When the sorbet has frozen (it will still seem quite soft), transfer to a storage container, cover tightly and freeze until ready to serve. The sorbet will solidify in the freezer.