You could insert a Section break right at the start of the document, with the watermark attached to that Section. If you then apply 'filling in forms' protection to just the first Section, recipients will be able to edit other Sections.

Alternatively, you could apply 'No changes (read only)' protection to the entire document, marking everything except for any Section break and the final paragraph break as exceptions.

Both approaches also limit your editing abilities in other ways (e.g. no use of Track Changes), so you'll need to experiment to determine which approach bet meets your needs.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]