1. In 2-quart saucepan over medium heat, heat water and sugar to boiling; boil 5 minutes, stirring occasionally. Refrigerate sugar syrup until chilled, about 2 hours.
2. In blender at medium speed, blend raspberries, liqueur, lemon juice, and salt until smooth. Press raspberry mixture through sieve to remove seeds. Stir into cooled syrup; pour mixture into 8" by 8" baking pan. Cover pan and freeze, stirring occasionally, until firm, about 3 1/2 hours.