For ease of installation AND for something that is all purpose rated for skiiing and tubing, I strongly second the Turbo Swing. However, if you specifically want a bar back there, I would make sure that whatever you get is a) rated for skiing AND tubing (big difference), and b) installed to the standard needed to support both.
Most ski bars are only rated for the pull of a skier, and not tubing. Also, quality installation to support the force of tubing is worthwhile. If adding one, mighty as well insure its safe and durable for all water sports activities IMHO. Costs a little more, but worthwhile in the long run.