In a medium bowl combine ketchup, water, dark brown sugar, cider vinegar, chili powder, garlic powder, onion powder, dry mustard, smoked paprika, cumin, and allspice. Stir until well combined and smooth.
Add chicken breasts to the bottom of the slow cooker. Pour the prepared BBQ sauce over the chicken breasts. Cover and cook on low for 4 -5 hours or until the chicken is fully cooked (the internal temperature has reached 165 degrees).
Remove the chicken from the slow cooker and shred each chicken breast using two forks. Return the chicken to the slow cooker and mix with the sauce.