@BraceMaker Do you mean for correcting the overall position of the course after it has been mapped by GPS in the usual way? If so, then I suppose it might well be possible to monitor a reflector mounted on a buoy or two. That said, to get changes in bearing and distance you would need a Total Station (or two) which is expensive....but at least the amount of kit lost when hit by a boat/skier would be less 🙂
Regarding course straightness, if the cable(s) are basically straight lines then as you say, there is always the propensity for the course to bow. However, there are designs out there where a series of "spreaders" and "tethers" going between the longitudinal wires effectively introduce an element of triangulation into the structure in much the same way that such members increase strength in towers (eg high tension cable towers or crane towers). This effectively makes the course more rigid at least in the horizontal plane. I will try and post some diagrams over the weekend to illustrate.
If the course is fairly close to shore on both sides, then it is not hard to run cross course tope tethers to the shores and they don't need to be under much tension to work. They do this at Thorpe Park in the UK.