A single watermark is most common, and mainly serves as a notice that the image is copyrighted. A single watermark is often easily removed with editing, like photoshop's content-aware fill. They tend to be a semi-opaque grey in order to not ruin the picture entirely, while being visible enough to serve their purpose.

An important point is that an image is copyrighted at the moment it is taken, and this is unaffected by the presence or absence of a watermark. So adding a watermark does not increase your legal protection.

They can also be used as a branding tool to advertise a photography business, as opposed to trying to discourage misuse of the image. A name and website so anyone who likes an image they see online can search and find the photographer.

Some agencies, like this one for example, cover their images with their watermark making it very difficult to remove.

Apart from visual watermarks, there is also digital watermarking, which is quite a different thing. This is not visible to the eye, but is embedded in the image (the best analogy I know if is to compare it to digital noise in an image). A lot of information can be encoded into the digital watermark. For example, if you licensed an image to several clients, you could embed unique IDs in each copy so that if they were used elsewhere, you would know which of the images it was.