I'm having an odd problem with this too.
The watermark text is stored in the gallery database that you set up when you installed the gallery:
HOWEVER: I have changed the text in that field to include a © (Alt+0169) mark, and it takes the change and shows the symbol as it should, but even with this applied, the watermark will not update on the images. Have closed browser, cleared the cache and still the old watermark is applied.
Am still pondering . . .
Reuploaded image, and new watermark took, but © comes out wrong, despite being right in database. Page encoding?