Watermarking might not be enough to prove that the person doing the distribution is the same one responsible of the leak. I fear at most it will be a contractual dispute between the person who received the watermarked file and the original distributor without the ability to easily link the overall counterfeiting charges.

But anyway, I don't think they really need to do that. They just need to shutdown any unauthorized distributors that make things too easy. As long as the friction introduced can convince people to pay a low subscription fee, they will be fine.