I don't see any way to do this in a text (Writer) document itself - at least, if you use the Background facility to insert the watermark. If your watermark is a graphic, you probably need to shade it some other software before inserting it.
If you need a text watermark - such as "Draft", perhaps? - try this:
o Start a drawing (Draw) document.
o Click the Text icon in the Drawing toolbar and drag a text box in your drawing.
o Type the required text.
o Adjust size, orientation, and so on - including perhaps setting the text colour to a pale grey.
o Right-click the text box and go to Convert > | To Bitmap.
o Right-click again and go to Save as Picture... to save just the required item (not the complete Draw document) in a suitable format.
o In your text (Writer) document, go to Format | Page... | Background.
o For As, select Graphic and browse to your saved watermark.
I trust this helps.
Brian Barker