One watermark, or a repeating tile across the page
For a firm mark such as DRAFT or CONFIDENTIAL, a single large watermark in the center rotated 45° is usually enough. To make it harder to remove from view, choose tile mode, which repeats the watermark across the whole page.
Set the opacity to around 20–40% so the document text stays comfortable to read. The watermark is drawn on top of the page content, so too high an opacity will cover the text underneath.