You can use Word Art, remove the lines, color with 25% gray, click
semi-transparent (or in latest version set the transparency to your desire).
Rotate to desired angle. On the screen, when you type over the mark, it
looks a bit fuzzy but prints out wonderfully
If you have many to do try this:
Here is some code from L. Howard Kittle.
In this Zip file you can find 4 example workbooks with code to insert watermarks in to your pages.
1 : Watermark Horizontal LANDSCAPE
2 : Watermark Horizontal PORTRAIT
3 : Watermark Vertical LANDSCAPE
4 : Watermark Vertical PORTRAIT
You can download it here.
http://www.rondebruin.nl/files/Watermark.zip