The quality of any final printed watermark will depend upon the quality of the medium on which it is printed: a smooth-surfaced medium will work much better than a rougher one. The general rule is that watermark should be printed in high-quality print and on high-quality media.
For example, printing on a course textile is likely to result in a poorly readable image, whereas printing on a smooth paper or plastic surface will maintain image quality and achieve a higher level of readability. Also, watermarked images do not work well on colored paper stock.
Always test with your chosen paper stock before distributing an ad.