Carefully remove outer green layer from watermelon rind using a vegetable peeler. Reserve remaining watermelon for another use. Cut rind into 1/2-inch pieces. Bring 6 cups water and 5 teaspoons salt to a boil in a large saucepan over medium-high heat. Add rind to pan. Reduce heat, and simmer 15 minutes or until crisp-tender. Drain rind. Place in a large bowl.
Place pickling spice, ginger, cloves, allspice, and cinnamon on a double layer of cheesecloth. Gather edges of cheesecloth together; tie securely. Combine cheesecloth bag, remaining 1 teaspoon salt, sugar, and vinegar in saucepan; bring to a boil, stirring until sugar dissolves. Pour hot vinegar mixture over rind. Cool to room temperature. Cover and chill 12 hours.
Strain rind mixture through a sieve over a saucepan; return solids to bowl. Bring liquid to a boil; carefully pour over solids. Chill at least 8 hours before serving.