Mix 4 caps diced watermelon, 1/4 cap lemon juice, 1 cup sugar, and 1/8 tea spoon salt together; chill 30 minutes; force through a sieve into a bowl.
Soften 1 envelop unfavored gelatin in 1/2 cup cold water in a small saucepan.
Set over low heat until dissolved, stirring constantly.
Stir into watermelon mixture.
Turn into refrigerator trays and freeze until firm, stirring once.