Hello skeptic. We try not to jump into the Club Bennington banter, simply monitor it. However, we will try to address your questions for the good of all!
Bennington offers ski tow bars and ski pylons, depending on the stern configuration. Both tubing and skiing are fine with our tow bars and pylons, and as you can see from our website, multiple tubes are fine, too. HOWEVER, you should never haul an empty tube. Inertia, an empty tube and high horsepower can cause a ski tow to bend. If the tube is big and flat, it may get dug below the water, which will act as a sea anchor while the boat is at speed, usually in a turn. You'll certainly feel the boat decelerate. If you pull off the throttle quickly, the pylon or bar will be fine. If your pylon or bar becomes bent so it is unusable, Bennington typically replaces them free of charge. (Always depends on the circumstances!)
DO NOT pull parasails or other boats with a ski tow bar or pylon!
Now, on to Sharkhide. Sharkhide will not hurt aluminum. It does a nice job of keeping the tube looking clean for several years, at least where fenders and trailers do not rub hard on the tubes. If you leave the boat in the lake year-round, the initial application may not last as long. Touch-up and maintenance to Sharkhide years later can be quite involved and we do not have experience with this. Sharkhide is toxic in the liquid form and can be difficult to apply evenly. Please contact the manufacturer directly for more information.