Re: Rope too short
In answer to your question, they make ski ropes with removeable sections but unless you have one of those, your option is to tie in a longer section or get a different rope. The knot you should use depends on whether you ever want to take the section out again. A double fisherman's knot or a water knot are two options, but once you apply the force of a tube or skier, its pretty much there to stay.
With that said, after reading multiple posts on the forum about towing a tube with a wakeboard tower, I'd recommend NOT towing the tube with your tower and use the ski eye instead. People advise that the tower is not designed for such forces. Just thought I'd pass it on.