Well, if there's no watermark but it looks like a professional photo, they still may refuse to print it, operating under the assumption that the photographer owns the copyright and not the client. Copyright is automatic--it does not need to be explicitly stated with a watermark or EXIF metadata. The most obvious thing to do would be to include a signed letter (in physical and electronic formats) with the disc, stating that you permit the holder of the disc to reproduce the images for personal use. In the letter, provide your contact information in case there are doubts.


You can explicitly grant a non-exclusive copyright to your client. This would still permit you to use the photos in your portfolio, for example. I do not recommend releasing it into the public domain--this would mean *anyone* can use the photo, which may not be desirable from your client's perspective.