I just read an article about 5 Useful Free Plugins for Gimp. One plugin is called Hidden Watermark with a couple of sample images. If you look at the original image and the watermarked one, you can’t really tell difference, until you open up and verify the image. I thought the purpose of a watermark is two-fold: to copyright your images, and to display credit.
The Hidden Watermark plugin covers the first, but not the second. Without a visible watermark, anyone can unknowingly infringe on your artwork. I rather have a visible watermark. I know I’ve stopped using images because I’ve seen watermarks on them. I’m sure others do the same. All the more reason to leave a visible watermark.
Just a personal preference.