Place half of the thinly sliced onions in bottom slow cooker; add pork and water, along with remaining onion slices.
Cover and cook on low for 8-10 hours. Drain liquid from slow cooker, shred meat and place back in cooker. Add barbecue sauce and chopped onion. Cover and cook on low for 4-6 hours longer. Stir occasionally. Serve with warm split buns and coleslaw.