Preheat the oven to 450 degrees. In a bowl, combine the orange, lime, and grapefruit juice and whisk together. Wash and pat the pig dry with paper towels. Sprinkle inside and out with the salt and pepper and place it in a large roasting pan, belly side up. Place the grapefruit, oranges, limes, onion, garlic, bay leaf, and thyme inside the cavity and skewer it closed. Turn belly side down and place a ball of wood or a rolled up piece of aluminum foil about the size of a lemon in the pigs mouth. Skewer the legs into position by pulling the forelegs forward and bending the hind legs into a crouching position (this will help a large pig fit in a home oven, if it fits already, this is not necessary). Cover the tail and ears with small pieces of foil to prevent them from burning. Place the pig in the oven and baste with the citrus juice mixture. After the pig has roasted for 15 minutes, baste it again with the citrus juice mixture and reduce the heat to 325 degrees. Roast for 20 minutes per pound longer, basting generously every 15 or 20 minutes with the juice mixture and then the pan juices. To test for doneness, prick the thigh with the tip of a sharp knife to see if the juices run clear. The internal temperature should read 165 degrees on a meat thermometer inserted into the thickest part of the leg. Remove from the oven and allow to rest for 10 minutes, loosely tented with a large piece of aluminum foil. Distribute the parsley and watercress sprigs loosely around the edges of a large warmed oval platter. Halve the trimmed blood oranges and place them around the edges of the platter, nestled in the greens. Remove the foil from the tail and ears and replace the wood or foil from the mouth with a lemon, lime, or apple. Carve at the table, with confidence.