Preheat Oven: Set the oven temperature to 400°F.
Prepare Pork: Use a sharp knife to make shallow cuts across the pork shoulder's fat cap. Be careful not to cut into the meat itself.
Apply Mustard and Salt: Rub the entire pork shoulder with the mustard, ensuring it gets into the slits. Generously season the meat with sea salt.
Make Spice Rub: In a small bowl, mix together the garlic powder, chili powder, cayenne pepper, black pepper, and celery seed.
Coat Pork: Evenly coat the pork shoulder with the combined spice mixture.
Roast: Pour the water into the bottom of a roasting pan. Place the seasoned pork shoulder inside the pan. Cover the pan tightly with foil or a lid.
Cook Until Tender: Roast for about 3 hours. The pork is done when the meat is very tender and the bone can be easily removed.
Finish: Take the pan out of the oven. Allow the pork to rest briefly before shredding or slicing it as desired.