When training for the sport of water skiing you are going to use many of the same principles that you would use if you were training for this activity on snow.
The main different however is that with water skiing you will need to perform a great deal more core work since the surface of which you are exercising on is a lot less stable than when you are on snow.
In order to achieve this, focus on performing a variety of different strength training activities on a bosu ball. This is a piece of equipment that looks like half of an exercise ball mounted on a flat surface. It will be very beneficial in your training for the sport of water skiing.
You can perform any exercise you would normally perform on the floor on this bosu ball such as bicep curls, squats, or overhead dumbbell presses. While performing the exercises try and keep your abdominal muscles tight while thinking of squeezing your bum muscles to keep your back in proper alignment. If you find that your back is starting to take on a curved appearance it must be corrected immediately otherwise you may end up experiencing back pain. Furthermore, be aware that while performing these exercises you will likely have to lower the amount of weight you are lifting slightly due to the reduced base of support you will be experiencing.
You may also want to focus on performing exercises that work the bicep and shoulder muscles as these will aid you in holding onto the handles and supporting yourself while out on the water. If you are lacking upper body strength you will not be able to go as long without fatiguing.
Implementing a good set of exercises to prepare yourself for this sport is an excellent way to improve performance and ward off injuries.