These watermarks only appear a few times in places like beside the page number. You'll have to provide a darn good explanation to convince me it makes the book harder to read.
And yes, it's still trivially copyable, but that makes it better in any way than DRM: it can be traced back to the original owner but doesn't make it unnecessarily hard to read it on multiple devices.
If your watermarked version appears online it is an indication that you might have uploaded (or not properly protected) it. This might not be enough to prove this in a court, but enough for the publisher's lawyers to annoy you.
But hardly any uploader would keep watermarks in, which makes them more of a way for faithful users to not give the book I.e. to friends since there is a visible reminder about being your personal copy.
An uploader can easily edit the watermark out of the book, if he is so inclined, and he probably is. It only annoys those who were not bent on uploading it in the first place.
Also, I specifically would not abide to any kind of law saying I cannot give a book I bought to a friend.
Many readers can't (they could learn it though) enough however indeed can.
That's why, from publisher's view, the second point is so important: Remind readers all the time that "it" (without going into details) is illegal so they continue buying.
It's not that kind of watermark. Rather, usually at the beginning, there'll be some text saying who bought the book, along with some kind of identifier. The id can also be hidden inside the file.
The books I've investigated had a text saying "This copy belongs to: %s" where %s was an email address at the beginning, and an invisible hex string at the end of every chapter. The string had a width and height of 1px, so most conventional readers didn't show it at all.
These aren't visual watermarks like what you'd get on physical paper. They just alter the digital copy so that it can be traced back to the buyer, and people are deterred from sharing it on the web. It's a very elegant solution if the 'watermark' can be made hard enough to remove, and I can see quite a few workable approaches.
