Photographers use watermarks to protect their work from image theft. But there is situations where watermarks can do more harm than good like going overboard with them.
More than one watermark, large or colored text, watermarks that run through the center of the image, basically any watermark that is too prominent distracts from the image and can actually hurt business.
People who buy photo are less likely to want to license an image with overly distracting watermarks. They may think you are a difficult person to work with.
Your images are less likely to be shared by others via social networks especially those that already link back to your work.
Building excitement about your work is important for your business and subtle, non distracting watermarks are more likely to do that.
Fact is, someone who wants to steal your work will. It’s not hard to remove a watermark. So overdoing it on your watermark isn’t going to really do anything but ruin your awesome image.