Developing the posterior chain of muscles (hams, gluts, erectors) is key to our sport. This is the group that is going to prevent you from getting broken when you take that big hit from the boat. Movements like deep (below parallel) squats, deadlifts, back extensions, reverse hypers, and my all time favorite the power clean (because it is a dynamic lift... more like waterskiing) will help develop this posterior chain and make you a more powerful skier and less prone to injury. Pull ups and dips are a great accessory movement to work in as well as some ab stuff.