Squats - Keep your back straight.  Try not to arch your back as you come up, and use only your thighs to push you up.  Do not go down beyond the point where your knees are bent at 90 degrees. 

Leg Press  Keep your back flat against the bench.  If you have trouble keeping you back flat, and your leg press machine allows it exercise one leg at a time.   Again do not arch your back and try not to use your hip for extra leverage.   Use only your thighs.

Leg Curls  Keep this slow and put as much, if not more effort into the downward movement as the upward.  Do not let the legs drop on the way down.   Move the legs back down in a slow and controlled manner.

Cycling/Stepper/Climber Machines  These will build leg strength and increase aerobic fitness

[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.