If "Allow editors to turn on/off watermark of the page" is disabled by default. When it is enabled in watermark settings, there will be a check box of “Display Watermark“ when editing a page .
...
Administrators are able to maintain the list of pages which do not display the watermark.
...
Click “Pages without watermark“ and check/uncheck pages in the dialog to hide/show its watermark.
...
Note：
...
When “Show watermark in attachment preview“ is enabled, if a page does not display watermark, neither do the preview interfaces of its attachments.
...
the editor can decide whether to display watermark on this page.
...