Pretty sure you have to have an observer, aside from that, you have to have to be able to legally carry the person you are towing on your vessel. So, if you had a two seater, one driver, one observer, that would still be illegal because there is not enough room for the person you are towing.
I've done some dumb things before, towing people in the boat with out an observer which was justified in my mind because we live on the lake and it was quiet out.
But, your getting bashed on potentially killing or severely injuring your brother by being reckless and coming very close to the docks. Even if the tube wasn't going to hit, any tuber know that when you fall on those sharp turnes your usually skipping across the water, in this case into a stationary dock. Not smart my man.
Aside from that, looks like fun but Ill save it for the boat or a supercharged sh!tdown lol