Maybe the watermark is being added by the printer driver? Click
on File | Print | Properties, and look for a watermark option
(for example, in my HP Laserjet printer driver, it's on the
Finishing tab).
Another possibility is that the watermark is anchored to a
paragraph in the body of the document and formatted as "behind
text", in which case you can select the watermark by clicking on
the Select Objects tool (the big white arrow) on the Drawing
toolbar, and then clicking on the watermark.