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.
A six-pound watermelon yields about two pounds of rind. Keep rind fully submerged in pickling liquid for best results, and leave the cheesecloth bag of spices in with the pickled rind so the flavor intensifies with time. Refrigerate for up to two weeks.