I am not sure which version of word you are using. I went to help and searched on watermark.
I was amazed it came up with this suggestion. Hope it helps.
Add a watermark to a printed document
To print a watermark, you must insert it into a header or footer. The watermark is printed wherever you place it on the page it doesn't have to be confined to the area at the top or bottom of the page.
1.
On the View menu, click Header and Footer.
2. On the Header and Footer toolbar, click Show/Hide Document Text to hide the text and graphics in the main document.
3. Insert a graphic a drawing object such as an AutoShape, clip art, WordArt, or picture or insert a text box.
How to insert a graphic
How to insert a text box
4. On the Header and Footer toolbar, click Close.
Notes
To view a watermark as it will appear on the printed page, use page layout view or print preview.
If the watermark interferes with the legibility of the text on the page, you can lighten the object you used to create the watermark. For more information, click
