Combine 6 cups of water with the salt in a pot and bring to a boil. Add the watermelon rinds and boil until slightly tender, about 10 minutes. Drain and set the rinds aside.
Combine the sugar, vinegar, mustard seeds, peppercorns, red pepper flakes, bay leaf, cinnamon and ginger with 1 cup water in a small pot. Bring to a gentle simmer, add the rinds and cook for 10 minutes. Let cool, then transfer to a pickling jar or a releasable glass container. Store in the refrigerator.