To prepare pork, at least one night before cooking, using a sharp knife or box cutter, deepen scoring on rind and extend existing scoring so that all of rind is scored. Be sure to cut all the way through rind and fat until you reach meat (do not cut through meat). Refrigerate pork, uncovered, for at least 12 hours and up to 2 days. Let stand at room temperature for 1 hour before cooking.
Preheat oven to 250°C (230°C fan-forced). Rub sea salt into scored rind. Season cut-side of pork with salt and place pork, cut-side down, on a rack set in a roasting pan.
Roast pork for 30 mins or until rind is crisp and golden. Reduce oven temperature to 160°C (140°C fan-forced) and roast for 2½ hours or until an instant-read thermometer inserted into centre of pork registers 65°C. Transfer pork to a carving board to rest for 20 mins.
Meanwhile, to make watermelon salad, in a medium bowl, whisk lime juice, vinegar, sugar, fish sauce and ginger. Stir in onion and set aside at room temperature for 20 mins to soften onion.
Place watermelon in a large shallow bowl. Spoon dressing over watermelon and turn gently to coat. Sprinkle with basil and mint.
Using a sharp knife, cut crackling from pork. Thinly slice pork and arrange with crackling on another platter. Serve alongside the Thai watermelon salad.