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.
