[BillSmithBooks]

I would argue that watermarking is NOT DRM for one simple reason:

DRM is designed to prevent you from copying or using on different devices without the express permission of the book's (song, movie) creator. It is intended to prevent format shifting or making multiple backup copies.

Watermarking prevents none of those things. It does however provide an incentive not the share your book with everyone on the planet for fear of having your account deactivated (in the case of Pottermore).

DRM is an (ineffective) content lock...watermarking is simply a fingerprint. Very different IMHO.