I never used to mark my photos with anything. However, a copyright attorney convinced me that it was well worth it to place at the very least a very small copyright notification on my images, which I now do, usually in the lower right corner, VERY small, like teeny-tiny, just enough to see it. I also color it so that it blends in pretty well with that corner of the photo to keep it from standing out like a shout, but can be readily seen.
It informs anyone who sees it that it IS a copyrighted photo, so they can't claim they didn't know. If they remove it, it shows intent to willfully violate my copyright. Those were the main points the attorney made, and they made sense to me. I also register every image that doesn't get deleted with the US Copyright Office, whether I intend to use it or not.
As for actual watermarks, I only use them on images posted to proof pages for my clients, in order to keep them from nabbing the images without paying. Those watermarks are big, they are ugly, they cover the entire photo from top to bottom with a repeating pattern of a copyright notice and "for proof only" in big fat letters. It's very light colored and very translucent, so that the client can see the images well enough to choose which ones they want printed at which sizes, but would be WAY too much work for anyone to successfully remove, and too ugly for them to use anywhere.
I don't post any other versions of those images anywhere until after the client has made their purchases.
My prints have no notifications or watermarks of any kind on them.