Arrange onion slices in an even layer in the slow cooker. Trim and discard skin and fat from pork. Place meat over onion. Pour in water. Cover and cook on low until meat is very tender, 7 to 8 hours.
Remove pork to a large cutting board and discard onion. Using two forks, shred pork. Strain liquid into a measuring cup to reach 1/2 cup (discard any additional liquid). Skim off fat.
Whisk ketchup with vinegar, molasses, Worcestershire sauce, garlic and onion powder and reserved liquid in a large bowl. Add pork and stir until coated.
Serve on toasted buns or over cooked rice if desired. Pork keeps well, refrigerated, up to 3 days.