I'm not an authority on this but I believe`watermark` is used when the artist posts a stamp on their artwork, so to speak. `Signature` would be more an artist signing their artwork, like a cursive name in a corner or something.
When in doubt, I'd use both tags. I usually don't differentiate them so I tend to use both tags to be on the safe side.
Also: not confirmed, but a watermark typically marks more of a picture than a signature would, from what I've seen.