Following up on this a bit more, the watermark should be anchored in
the header or footer of the document, and the article at
http://word.mvps.org/FAQs/Customization/ProtectWord2000PlusHeader.htm
shows macros that can "lock up" the header or footer so they can't be
selected. The fatal drawback of this scheme is that if you send the
document to someone without the template, they won't get the macros.
If you try putting the macros into the document itself, and the
recipient has Medium or High macro security, they can still prevent
the macros from running.
The bottom line is the same -- there's no way, really.