Put brisket in a large, resealable plastic bag; refrigerate 30 minutes to overnight.
Stir barbeque sauce, water, Worcestershire sauce, and liquid smoke together in the bottom of a slow cooker. Lay brisket into the sauce mixture. Arrange onions atop the brisket.
Cook on low until brisket is very tender, 6-8 hours.
Rest brisket 10 minutes before slicing or shredding.
Serve with sauce and onions from the slow cooker.