Mix paprika, pepper, salt, brown sugar, chili powder, and cumin together in a bowl; rub evenly over the surface of the brisket. 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 to 7 hours. Rest brisket 10 minutes before slicing or shredding; serve with sauce from the slow cooker.
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