This sweet-tart relish is a staple of the American South; it is particularly good with barbecued chicken or pork.
Using a sharp knife, slice watermelon in half lengthwise; scoop out flesh, and reserve for another use. Scrape rind with a large metal spoon to remove all traces of pink. Cut rind crosswise into 1-inch-wide strips. Using a sharp knife, peel green skin from rind, and discard; cut away any bruises or bad spots. Slice rind into 2-inch lengths.
In a large nonreactive bowl, combine the water and salt. Add watermelon rind; let soak overnight, covered, at room temperature. Rinse 2 or 3 times in fresh cold water, and drain well in a colander.
Heat sugar and vinegar in a nonreactive stockpot over medium-high heat, stirring until the sugar has dissolved. Add the remaining ingredients and the watermelon rind, and bring to a boil over medium-high heat. Reduce heat to a simmer; cover, and cook 30 minutes. Remove from heat, and let stand overnight at room temperature.
Transfer watermelon relish to jars or airtight containers; cover, and store in the refrigerator up to 2 weeks.