In a small bowl, whisk together the paprika, brown sugar, salt, black pepper and cayenne. Rub the spice blend evenly over both sides of the ribs.
Spray a 6-quart slow cooker with non-stick cooking spray. Place a layer of sliced onions on the bottom. Alternate layers of ribs and onions. Pour the water over the top of the ribs. Cover and cook on low for 7 hours; the meat should be extremely tender and falling off the bone.
Using tongs, remove the ribs to a large plate. Pour off all accumulated liquid and the onions. Return the ribs to the empty slow cooker and pour the barbecue sauce over top, turning the ribs so they are evenly coated. Cover and cook on low for an additional 1 hour. Cut and serve!