In large saucepan, bring 4 cups water and salt to boil over medium high heat. Add rind pieces and boil until tender, about 5-8 minutes. Strain. Transfer rinds to a medium bowl.
In saucepan, combine sugar, cider vinegar, allspice, peppercorns, cloves, pickling spice, celery seeds and gingerroot. Bring to a boil over medium-high heat, stirring until sugar dissolves. Reduce heat and simmer for 8-10 minutes, until slightly reduced.
Pour brine over watermelon rinds in bowl. Place plate over top to keep rinds submerged in liquid. Cover and refrigerate for one day.
Transfer to a glass jar and keep sealed in the refrigerator for up to 2 weeks.
***Cut watermelon pulp from rind, leaving thin layer of pink on rind. Cut green outer skin from rind; discard. Cut enough rind into 1 x 1/2-inch pieces to measure 2 cups.