A watermark is just about as effective to deter the casual user from distributing their books nilly-willy. I have DRM free ebooks which contain something like "

A watermark is just about as effective to deter the casual user from distributing their books nilly-willy. I have DRM free ebooks which contain something like "Thanks X Y for buying this book" and so it's blatanly obvious who it belongs to. Now, you could remove it but you could also remove DRM.