Sooooo, I've been saying this in security eSeminars and meetings for ages now.... Watermarks are a really useful tool in helping to keep your documents and other contenet secure.
Since most document loss is accidental... (I send you a document and you think the content is so great you share it with some who actually shouldn't really have access) ... If you watermark documents with the appropriate phrase, they're less likely to be accidentaly shared. It's easy to imagine how a large "Company Confidential" watermark might stop someone from inadvertently sharing a document with partners or customers.
The best bit? It's really easy to do, take a look here.....
....see I told you!