Actually using PS is pretty easy and you can make it into an action by creating a new action and then going through the following steps on one photo, then hit stop recording and then go to batch.
To create watermark: Pick the T (text) in the tool bar and then add whatever you want to write (initials, name etc) size it accordingly, and choose your font.
Then go to Layer, choose Layer Style, i like to add a drop shadow and also a bevel and emboss,
Then also in Layer style choose Blending Options, it gives you general blending and advanced blending with sliders, the play with sliders down to around 20 i would say or until it is at an opacity you are happy with.
Then just click on any other tool in the tool bar, go back to Layer and hit Flatten Image. Voila!
If you make it into an action i find it better to create two folders, one for vertical images and one for horizontal so things don't get messed up.
Mike