I have found Phatch searching for a watermark tool running on my linux machine this morning and it helps me making visible watermarks exactly the way I wanted them.
So I will be using Phatch as my standard watermark tool.
There is only one thing that I recommend: The exif information on the new pictures (those with watermarks) are very »limited«. If it is possible to apply a watermark and keep the exif information from the original pictures (camera, etc.) -> please implement it.
If I missed a checkbox or something else -> please inform me where I went wrong.