I was thinking the special tubing ropes were 65' long. Anyway it goes, pulling tubers is a very dangerous proposition.
We have had a couple of serious injuries on our lake where tubers had very light ski ropes tied to their tubes. In a tight turn, the tubers can double the speed of the tow boat, and the pressure on the rope is tremendous.
Unfortunately, when a rope breaks, there is absolutely no control over the tube and the riders. Our neighbor's 16 year old daughter broad sided a boathouse, and she was in the hospital for months. And she came out a different person.
By all means, protect your riders by using a very high test rope, preferably a 1/2" rope that will stand up to the rigors of tubing.