This would only work if you were personally selling directly to your end users, asking them to identify themselves first, and barring them from selling on to others. Or you would require your distribution chain to do this for you. Otherwise the guy four links down the chain might be the big "pirate" distributor. Ya, this is the problem with water marks. Its not scalable and only works for wholesalers and large customers, where investing on watermark will still be financially sensible. Anything that go to end users in masses, you will have to trust them to be ethical and be open to some copying being part of doing the business.
I think this can only be done by operating system, not an after market application. There was time Microsoft seem like they would have managed to implement such a DRM, but Apple became dominant and they refused to go along with them. Since such DRM can only work if implemented across all the platforms, I doubt it will ever be possible. Regards, William