My recommendation actually, regardless of whether you use watermarks or not, is to always keep multiple copy types of each page!
I have as follows:
- A blank, full size original with only the artwork and no speech bubbles or text. In case it gets translated or I ever need to pull an image for promotional purposes.
- A Bleed image. This is the full size original with speech bubbles and text, intended for print.
- A cropped (since my pages stretch to the edges they require full bleed, so I crop whatever parts of the edges I don't want for web) downsized version for online posting. This is the one you could put your watermark on!
This saves you a lot of trouble when it's time to put together your PDF for print!