Combine the watermelon, lime juice, sugar, salt, chile powder, and cayenne in a blender or food processor and blend on high until smooth. Pour the mixture into a 9 by 13-inch glass or metal dish and cover with plastic wrap. Slide the dish into the freezer and chill for one hour. Remove the dish from the freezer and scrape the surface with a fork to break up the crystals, cover, and place back in the freezer. Scrape the granita every hour or so until it is completely frozen and crystals have formed throughout. Serve in clear glasses topped with a pinch of salt and a squeeze of lime. The granita will keep for about five days, covered, in the freezer.