As Paul said, the Watermark feature is all or nothing.
Watermarks in Ribbon Versions of Word

If you want a different watermark on every page, create the watermark (image) and insert it into the page itself rather than in the header. It should be set to be behind text and placed relative to the edge of the page.

00 deleteme 1.png

This should be done when you are otherwise done with editing because images in the body of the document (rather than in the header) are going to be anchored to text and if the text moves, the watermark (image) can move as well (to a different page).

Note, placement in headers is preferred for a number of reasons but you do not want a new section for every page (or if you use all of the headers, every three pages).