I wakeboard(badly), waterski(pretty well for an old guy) and freestyle jetski(my true love). I do a lot of leg work as I head into water ski season..squats, lunges, leg presses, calf raises. I hit my shoulders a lot also, because the standup(jetski) can really take a lot out of you in that area. Arnold presses, lateral raises, military presses, etc. all help, and wil help for waterskiing too. Depending on your own fitness level, you may want to throw in some wrist and forearm work. My arms are in decent shape, but my forearms would still tire from a long day of waterskiing. I now use a 17 degree radiused handle on my ski rope and I wear gloves, and that made a huge difference, as far as not getting 'arm pump' in my forearms while getting dragged behind the boat.
