How to Make It
Using a vegetable peeler, remove outer layer of green peel from watermelon; discard peel. Cut watermelon in half lengthwise, then cut in half crosswise. Cut each quarter into 1/2-inch-wide strips. Using a paring knife, remove pink watermelon flesh from rind, leaving a thin 1/4-inch layer of pink flesh attached to rind. (Reserve removed watermelon flesh for another use.)
Place rind and salt in a medium pot; add water to cover by 1 inch. Bring to a boil over high; boil until rind is tender, about 3 minutes. Drain; set aside until cool enough to handle, about 5 minutes. Divide rind evenly among 2 sterilized 1-pint jars.
Place vinegar, sugar, garlic, pickling spice, allspice, peppercorns, and 1 cup water in a large saucepan; bring to a boil over high. Boil, stirring often, until sugar dissolves, about 3 minutes. Pour over rind in jars; let cool to room temperature, about 1 hour. Seal and refrigerate at least 24 hours or up to 2 weeks.