[Alisa]

Yep. That's why I said "most" of the fair uses. A hypothetical watermarking system would produce a file that was likely not transferable by the owner without risk. So you might lend it to a friend who you knew wouldn't screw you over by uploading it, but you couldn't sell it on your own. I'm ok with that personally as long as the price is right. As far as preventing, no, it wouldn't prevent them but it would allow for enforcement which is where I think anti-piracy efforts should be concentrated.