A very quick comparison of the text with a hardback and the aformentioned (cough) unauthorised ebook shows that the text is extremely clean, although I haven't gone to far as to check the site that lists the various corrections that have been made over the years.
The 'watermark' appears in the ePub on the copyright page, and is incorrectly tagged (someone didn't check what was already there, I think!) and looks like this:
<p><p>This book is watermarked and was acquired by user [32 hex digits] on 27 March 2012</p></p>