This is internet, if someone wants to remove your watermark and claim the image as their own they will do it, and you won't be able to do anything about it. There is no point combating this with watermarks of any kind because you would need extremely large and obnoxious watermark to stop people from "stealing" your art, which would defeat the entire purpose. The upside is that there are many of us who frown extremly upon art theft, and with the help of reverse image search we have actual tools to combat it, by making people aware that someone "stole" something. Your work is safe as forever being your work, merely by the fact of being uploaded to DA at the particular date, and when someone looks up the image to find the real source, they'll be able to find it, and that's really all that matters in the end, making it possible for people that care to track your work back to you so they could see more, and that's already possible due to active communities and reverse image search engines. There is no need for watermarks, for they only degrade your work and avert attention. I too used to put signatures in my DA works, 7 years ago.