DRM or not?
A watermark can be deleted, if the user takes some time to open the book in an editor, or devise some conversion setup that removes the watermark. Deleting a watermark, if it's not an obnoxious one, is a deliberate act to modify the bought ebook.
DRM can be deleted, just by running a readily available little program, no further knowledge required. Removing DRM is probably routinely done by anyone who buys ebooks (and who knows that it can be done), and it's not necessarily indicative of a dishonest behaviour, but rather a means to guarantee they can give the book to friends and relatives and use it in future readers.
So, I'd say DRM is much more likely to be removed than a watermark. None of them will prevent your book appearing in the darknet some day, only scarcity of available copies and lack of interest will