One of the biggest debates that occur between members of the photography community involves watermarks. You see, watermarks are an important part of the photography process because they help you maintain the rights to images you have taken, but there is a large section of the photography community that takes umbrage with the practice of using watermarks. They feel like it compromises artistic integrity and turns the art of taking photographs into a purely commercial endeavor.
If you are looking for a personal opinion, mine is that watermarks are a good idea if they are simple enough to not overshadow the image itself and not appear completely obnoxious. The reason that I feel like this is a good idea is that people steal images on the internet with alarming frequency. By adding a watermark, you can at least ensure that you would know when and where your images are being used. Artists have been adding signatures to their work for hundreds of years, this is a similar kind of signature for you.
Now, with that being said, it is important to understand that watermarks can often be incredibly obnoxious. A lot of photographers add watermarks that are very ostentatious to the point where you would get distracted from the photograph itself because the watermark obscures it in some way. All it takes is putting some thought into your watermark instead of applying one randomly.
One thing that you should keep in mind is that black and white images are particularly dangerous to add watermarks to. This is because black and white photography is all about subtlety and style, it is about using shadows to accentuate certain objects in a way that would ensure that all attention is placed on the right ideas. Nothing would destroy that kind of dynamic more than a watermark because it is by its very definition something that draws attention to itself. The most common mistake that people make here is that they add colored watermarks to their images. This is terrible because it destroys the black and white aesthetic of your art.
Basically, what you need to do is create a watermark that is visible but not overly so. A good way to ensure that your watermark is as subtle as possible is to make it fairly translucent. Opaque watermarks are the dictionary definition of tactlessness. When you have the opportunity to make things visible without ruining their overall quality, why would you do something as silly as making your watermark opaque? Besides, art has a way of getting recognition without a watermark.