Watermarks ruin the artistic nature of sharing your work with others. Why share it if it's ruined with a huge stain in the middle? Sure, you ensure that it cannot be stolen. But you also ensure no one can appreciate your work properly. The only way to "unlock" it is by giving you money? It's very speculative in my opinion.
I think I can relate to the sensations of rage that must be felt when someone puts your work somewhere and takes credit for it. So I do not mean any offense with my comment.