That's a clever idea but do companies accept watermarked personal documents? Especially ID card or passport? Also, isn't it easy to remove watermark via photoshop? Especially with some AI properties.
I always try to place the watermarks in places that the receiver still is able to read the necessary information (Numbers and names mostly), but make it so that the watermark is in an area that would make its removal very easy to spot (or even harder). I know that if someone is really interested in copying my information will eventually find a way to do it, but I find that it works great as a deterrent for most scenarios.