Good watermarks are meant to be invisible to the naked eye anyway, or "mostly". The point of the watermark is that it does not disturb too much the data contents, but the presence of the mark can still be detected. In many bank notes, the watermark is made visible to the naked eye because its presence must be detectable by a vendor who will use, indeed, his naked eyes. However, it is better if the mark can be somehow "hidden" so that it does not degrade the quality of that which is marked, especially when the marked element has some aesthetic value. In that sense, good watermarking, especially for pictures and videos in the digital age, should be "steganographic".
In fact, if it is interesting for anybody to remove the mark (e.g. to distribute copies which cannot be traced to him), then the mark ought to rely on steganographic techniques so that its presence is not detected.
Unfortunately (or not, depending on point of view), it is nigh impossible to do user-specific watermarking that cannot be removed. Indeed, if two users purchase the access right to the contents, a simple comparison (byte by byte, pixel by pixel, frame by frame...) will reveal where the user-specific watermark lurks, and allow removal. Watermarks have been studied a lot in order to enforce DRM; the rough summary is that it does not work. The hope was to be able to add a watermark that users cannot remove. Nobody found a method for that. At best, you may add some data whose presence the user does not even suspect, but the digital comparison between versions for two users destroyed that hope.
It used to work in the analog age; for instance, video-tapes containing copies of new movies, sent to critics one week before the theatrical release, were heavily tagged (in the analog signal) with an identifier for the target critic.
The only successful application that I can think of is the story of a teacher who voluntarily pushed (obviously) fake information in Wikipedia so as to trace within his pupils who were stupid enough to copy&paste from Wikipedia without even trying to understand what they were doing.
(The teacher and the powers-that-be at Wikipedia then engaged into acrimonious bickering and all failed to draw the proper conclusions. The teacher wanted to prove to his pupils that "the Internet" is unreliable; the Wikipedia people vindicated the teacher for "vandalism" but really for the capital crime of suggesting that Wikipedia is not the most reliable of sources. The only correct inference was that of the pupils: if you want to cheat, do it well. Don't forget that teachers are nasty cunning buggers.)