Cut ribs into 2- or 3- serving portions; place in 5 to 6 quart slow cooker. Sprinkle with salt and pepper. Pour water over ribs.
Cover; cook on low setting for 8 to 9 hours.
About 1 1/4 hours before serving, remove ribs from slow cooker. Drain and discard liquid from cooker.
In a large bowl, combine molasses, mustard and vinegar; mix well. Dip ribs in sauce to coat; place in slow cooker. Pour any remaining sauce over ribs. Cover; cook on low setting for an additional hour.