1 large large watermelon
1 (750 milliliter) bottle spiced rum (such as Captain Morgan’s®)
Cut a 4- to 6-inch-diameter circle, about 6 inches deep, in the middle of the watermelon using a paring knife, making a plug you can pull out. Slowly pour spiced rum into the watermelon; wait until it is absorbed. Repeat several times until spiced rum is used up; this can take several hours.
Replace the plug in the watermelon and wrap entire watermelon in plastic wrap or aluminum foil; refrigerate for 2 to 3 days.
Remove plastic wrap and slice watermelon.