Cook cranberries in the 1/2 cup water in covered saucepan until skins burst, about 6 minutes, and remove from heat. Stir in sugar. Add tapioca and salt to boiling water and cook over low heat, until tapioca is clear, about 5 minutes. Add cranberries and cook 2 minutes, stirring occasionally. Pour in serving dish. Chill. Unmold and serve with whipped cream.