i guess there are multiple issues here.
1) watermarking that is just meant to add a VISIBLE extra overlayed image, usually to corner of picture, or covering entire picture, which is not intended to actually hide information but rather brand your image.
2) steganographic watermarking, designed to secretly embed information that you could use to prove ownership etc., which would be invisible and hard to remove.
i was thinking more along the first lines. for the 2nd case configuring an external tool would be your best bet probably since as fodder says it can get a bit involved and probably doesn't belong inside screenshot captor itself.