Stomach and lower back  - it is important to have a strong core.   The basics are the best here.  Sit-ups and crunches are the best thing for building a strong stomach.  For a bit of variety you could also do leg raises.   If you do leg raises ensure that you lean back on your elbows, with your hands level with your waist, but do not hold the small of your back.  The most effective way of doing leg-raising exercises is to move the legs from 6 inches off the ground to a 45-degree angle.  Keep going as long as you can.  Do not let the legs drop to the ground.   You can also use crunch machines and to exercise your stomach, but do this to complement sit-ups rather than instead of them.  Ad-trainers are good, but only if they are used properly.  If in doubt stick to sit-us. Try for between 50-100 per day.

To complement your stomach muscles you need to have a strong lower back.  This can be aided by doing back curls.  Make sure that you do these slowly and in a controlled manner  do not jerk the back it WILL cause injury.

Something else that is good for both the stomach and the lower back is rowing  but you need to make sure that your are in the correct position.

[AEROBIC FITNESS]   [LEGS]   [MIDRIFF]   [UPPER BODY]   [ARMS]     

For a full training guide including day by day waterski fitness program take a look at the
Complete Guide to Water Skiing.  The book also includes advice on diet and loads of
on-the-water training tips and drills.