You can also place your watermark design on the master layer and it will appear in every page if you're using a multipage layout. It the master layer is over the guideline layer, the watermark will appear on top of everything. If you drag the master layer below the guidelines layer, the watermark will be under everything in the pages.
At least theoretically...
I just tried it on CD11. The object seems to keep jumping back to regular layers after switching to different pages... http://www.talkgraphics.com/images/smilies/confused.gif. Anybody can confirm this bug or is it just me?