Watermarks can be made to be pretty robust, though having an open source detection routine will make it much easier to remove (if nothing else, if you can differentiate the detection algorithm then you can basically just optimise it away). The kind of watermarking that is e.g. used for 'traitor tracking' tends to rely on at least a symmetric key being secret, if not the techniques used as a whole (which can include more specific changes to the content that are carefully varied between different receivers of the watermarked information, like slightly different cuts in a film, for example).