Depends on what kind of watermark. If you just want to add some text, Irfanview has a very powerful batch feature, which includes adding text, you pick the location, font style/color, transparency, etc, etc.
If you want a "photo" watermark (a graphic you create), the hardest part, from my experience, is actually creating that - it needs to be transparent, for example.
I have all mine added by my web gallery software, so I don't do anything on my own PC (apart from create the watermark itself.)