You should enable "Flatten PDF pages to images" with watermarks

[paulchaum]

If you don't enable it, the watermarks can be removed with a PDF editor (like PDF Text Editor in Stirling for example).

You can enable it in Formatting > Flatten PDF pages to images. This will turn each page into an image, so there is no underlying selectable/extractable text left.

Maybe this option should be selected by default, like with the Redact feature?