Place watermelon cubes in a blender or food processor. Cover and blend or process until smooth. (You should have about 3 cups of watermelon mixture.) Stir in sugar.
In a small saucepan combine the cranberry juice and gelatin. 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 3 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 1 to 2 hours more or until firm.
To serve, let stand at room temperature for 5 minutes before scooping. Serve with watermelon wedges, if desired. Makes 8 (1/2 cup) servings.