kvogell,
The only way I know to do it is to print it out and then set a damp
glass on it
Seriously though, I'm not sure why anyone would want a watermark on a
project plan. This is more a feature available in publishing or text
type applications and it is not directly available in MS Project (at
least as far as I know - I don't have the server or professional
versions). You can simulate a watermark by using the drawing tools
(Drawing toolbar) to place a watermark graphic in the Gantt Chart
portion of the view. Note that you will need to replicate the mark on
each portion of the view that is printed on a new page. Not necessarily
a convenient approach but it will give a reasonable facsimile of a
watermark.
Hope this helps.
John