1 (8-lb) piece watermelon (flesh and rind)
1 1/2 cups cider vinegar
1 1/2 cups water
2 cups sugar
1/4 cup minced peeled fresh ginger
2 tablespoons minced small hot green chile, such as Thai or serrano (including seeds)
1 1/2 tablespoons minced garlic
1 teaspoon salt
black peppercorns, coarsely crushed with flat side of a large knife
Remove watermelon flesh from rind and reserve flesh for another use.
Scrape off and discard any remaining pink flesh from rind, then cut rind crosswise into 2-inch-wide strips and remove green peel with a Y-shaped vegetable peeler or a sharp knife.
Discard green peel.
Cut white rind into 1/2-inch cubes (you will have 5 to 6 cups).
Bring rind and remaining ingredients to a boil in a 4-quart heavy saucepan over moderate heat, stirring until sugar is dissolved.
Reduce heat and simmer, uncovered, stirring occasionally, until rind is tender and translucent and liquid is syrupy, 45 to 55 minutes.
Cool chutney, uncovered, then chill in an airtight container 1 to 3 days to allow flavors to mellow.