Waterman ball point pens can range from $40 to $100. They are uniquely made, so your best option for repair is to go straight to the Waterman service station nearest you. You can also email them for information about repairs. There are some steps you can take, however, depending on the problem you are having with your pen.
Review your warranty. If you received your pen directly from Waterman, or as a gift, you may have received warranty information as well. According to the Waterman website, a new pen is under warranty for three years. However, this only covers manufacturer defects, and not accidental damage or normal wear and tear. So, if your pen has never worked correctly, you may want to contact Waterman about a replacement or comparable substitute before trying to repair it.
Check the ball roller. If your pen has been going long periods between use, it could be that dried ink has stopped the ball from rolling properly. Try moving the tip of the pen over the pad of your fingertip. If the ball is not rolling smoothly, add a few squirts of window cleaner to your hand and re-roll. This should loosen up any gunk hindering the ball's movement. Make sure to wipe any excess cleaning solution from your pen with a soft cloth to avoid damaging the surface.
Check the ink level. If you have been using your pen for a while, it may just be that you are out of ink. Ink refills are available directly from the Waterman website in a variety of colors. You may need to know the point size of your pen. You can also check local retailers; Waterman has links to fine pen stores that carry their products.
Check for physical damage to the outside of the pen. If the pen is writing finely, your problem is probably cosmetic. There are many suppliers that provide vintage pen parts, from washer clips to blotting paper. They can be replaced with relative ease if you have the correct tools, but for vintage pens you may want to consult a professional.
Any accidental damage caused by unauthorized repairs will void your warranty with Waterman, so when in doubt, go there first.
Wear gloves and old clothes to prevent ink damage when working with pens.