In a large bowl, whisk together the 6 cups of cold water and pickling salt. Place the watermelon rind in another large, non-reactive mixing bowl and pour the salt-water mixture over it. Add the ice cubes, transfer to the refrigerator, and let it sit for at least 6 hours or overnight.

Drain the watermelon rind and rinse thoroughly under cold running water. Transfer it to a large pot, cover with water, and bring to a boil. Reduce the heat to a simmer and cook until fork-tender, about 10 to 15 minutes. Do not overcook. Drain, and set aside.

Combine all the remaining ingredients in a large pot, and bring to a boil. Reduce the heat to a low simmer and cook for 10 minutes. Add the drained watermelon rind and cook for about 1 hour, or until the rind is almost completely translucent.

Divide the watermelon rind between 2 sterilized pint-sized canning jars and add enough pickling liquid to cover the rind by at least ¼-inch. Add a cinnamon stick to each jar. There should be at least ½-inch headspace. Wipe the rims clean with a damp paper towel, and attach the lids and rings. Process the jars in a hot water bath for 10 minutes.

Remove the jars from the hot water and let them cool. The jars should seal as they cool. Any jar that does not seal properly should be refrigerated and consumed within 2 weeks.

Watermelon pickles will keep, stored in a cool dark place, for up to 1 year.