I used to be a competitive waterskier. I am more than familiar with the sport to say the least.
Haven't skied in years but injuries ended my career a long time ago.
A couple of tips for this kind of photography:
If the skier is good enough shorten the rope some. Do 15 or 22 off.
Crank up your ISO to 400 or 800
Stop down to f8 or so.
Get your shutter speed up. 1/500 is ok but for really fast moving subjects you need faster than that. Try 1/1000 or 1/2000 and just play around a bit.
Try not to have them drive the boat with the sun to the skier's back. (you didn't do this but just saying as a general rule)
Also just like anything early and late light is the best.