1. Remove and discard peel and seeds from watermelon. Cut enough watermelon into bite-sized chunks to make 6 cups. In blender at low speed, blend one-third of watermelon chunks with confectioners' sugar, lemon juice, and salt until smooth. Pour mixture into 9" by 9" baking pan. Puree remaining watermelon chunks, half at a time, until smooth; stir into watermelon mixture in pan. Cover pan with foil or plastic wrap and freeze, stirring occasionally, until partially frozen, about 2 hours.
2. Spoon watermelon mixture into chilled large bowl; with mixer at medium speed, beat until fluffy but still frozen. Return mixture to pan; cover and freeze until firm, about 2 hours.