I always use watermarks on copyrighted material or trademarked material that I am trying to protect.
And you are correct, watermarks with a little effort can be removed but the signature that the watermark was part of the graphic, image or document can never be removed without an extreme amount of effort and someone very knowledgeable in binary machine code. It is all hexadecimal information that most people have no idea how to read.
Everything created on a digital system now has a digital signature, it has been that way for many years. Many people do not even know that it is there or how to see it. That signature includes a tremendous amount of information on the creation of that item. Whether it be an image, a photo, a document or whatever. When you add a watermark you add to that digital signature. Removing the watermark from view does just that, it removes it from view.
If the watermark is added correctly the digital signature does not remove the watermark information, removing the watermark just adds to the digital signature. The information it adds is kind of interesting. It can tell me the date, the application, the MAC address of the hardware and quite a bit of other information about how the watermark was removed. Ask any good copyright lawyer they use this type of information all the time to protect their clients.
So yes, if you have something that you really want to protect, watermark it. Or better still add your information on the copyright to the digital signature. Without a tremendous amount of effort that signature cannot be changed. Oh and by the way, even if the signature is changed, it tracks that also. And for all you guys that think, I will just remove the signature, If you remove the signature completely the document is no longer valid and would not load. Pretty cool, huh.....
I think I have given away enough secrets for today.