Add the watermark.
On the one page where you want the watermark, insert 2 continuous section breaks, one at the start and one at end of the page, or a page section break at the end, depending on your need (under insert -> break).
Double click on your header or footer (or go to view -> header/footer). A toolbar will pop up. Go to the pages before and after, then click the "link to previous" on both pages to deselect it. Delete the "Draft" water mark from the page before and after, and voila!
To do alternate pages, you may have to do this on every page :\
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