Watermarks can be placed on photos with a copyright notice and the name of the photographer, often in the form of white or translucent text. A watermark serves the purpose of informing a potential infringer that you own the copyright to your work and intend to enforce it, which may discourage infringement. It also prevents the infringer from being able to claim that they did not know the work was copyrighted.
Another important aspect of watermarks is that if they include information such as the name of the photographer, they may be considered copyright management information under the Digital Millennium Copyright Act (DMCA). If a copyright infringer removes this information, for instance by cropping out the watermark, they can be liable for additional damages, and these damages apply even if the copyright was not registered before the infringement took place.