Well, it is not free, but I use Lightroom.
I started using it for that, but also because I felt there was a 50/50 chance that Photos might be a retrograde step when introduced and in due time or with a new computer I might need to switch totally to a third-party product. The jury is still out on that one.
The bonus is that edited results for many pictures appear to be better in Lightroom. I copy pictures from iPhoto to an external drive, import them to Lightroom, export with the watermark back to the external drive then, if necessary, re-import them to iPhoto, deleting the original if I want to do that.
Needless to say, I watermark quite sparingly.
Well, you did ask...