You could go for bicep curls and tricep extensions, but the best things to do are the straight-arm chin-ups and tricep dips.  These will help to build a good size to strength ratio.

In addition to this you need to work on forearm and grip strength.    This can be done easily, by using a tennis ball or a sponge ball.  You can work on your grip while watching TV  just sit and squeeze the ball.

[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.