Our Jetta has two water separators. The main is a translucent  8" x 6" x 9"
box under the body just in front of the fuel tank. It has a large drain
valve, and small (1 cm) tube leading out. I recall the rec. svc. interval
to be monthly. In the spare tire well, a clear plastic bladder is provided
by VW for this task. If the separator gets too full of water, the glow plug
light will flash while you are motoring down he road.

The second (separator of last resort) is the fuel filter. It has a small
petcock at the base, and a vacuum-relief screw at the top.

In 8 years of ownership, I have only had water in these once; a time when
somehow A station had a REALLY bad water problem, and I got almost a gallon
in the tank. It stopped my car, and took a few hours of work with a
hand-held pump to pass the fuel through the separators and remove the
water. Other than that, these gizmos have been superfluous, for me.

G. Matthew Bulley
Bulley-Hewlett
Corporate Communications Counselors
www.bulley-hewlett.com
Cary, NC USA
888.468.4880 tollfree