Place watermelon cubes in a blender container or food processor bowl. Cover and blend or process until smooth. (You should have 3 cups of the watermelon mixture.) Stir in sugar.
In a small saucepan combine the gelatin and cranberry juice. Let mixture stand for 5 minutes. Stir mixture over low heat until gelatin is dissolved.
Stir the gelatin mixture into the melon mixture. Pour into an 8x8x2-inch baking pan. Cover and freeze about 2 hours or until firm.
Break up frozen mixture and place in a chilled mixer bowl. Beat with an electric mixer on medium to high speed until mixture is fluffy. Return to pan. Cover and freeze about 6 hours or until firm. Makes 8 (1/2-cup) servings.