I thought about that too, however you could create the thumbnails views etc. watermarked and move the original files into a different Album that is then visible to "special" user groups i.e. Media with no watermarks. Surely it is an additional step as you need to create the views watermarked and leave the originals intact, and you need to create duplicate albums with different access rights, one watermarked one without watermarks.
Certainly a watermark feature per album/user would be nice, and it would make life so much easier. But then one needs also to randomize the file names of thumbnail views and link those random file names to the original, non-watermarked files. Is it worth the effort and wouldn't it add to the load on the server? If you don't get rid of the direct link between original file name and thumbnail views it is possible to get the name of the watermarked picture and sort of reverse engineer the file name of the non-watermarked picture, which defeats the purpose of the watermark. And no, disabling right clicks is no solution, disabling "Save as" neither and neither is a solution against people with network sniffers filtering out the path and file names or just plainly viewing the source code of the generated HTML.
I guess the best and safest solution is seperated albums, even though it means more work on your end, IMHO a small price to pay.
Tom