If you have a Word Document based on a template with a watermark on it and you want to remove the watermark in the new document, calling the WordBasic RemoveWatermark method seems the easiest way to go: except it's a COM object so you have to use a bit of late-binding trickery..
string templateLocation = "c:\template-dir\template.dot";
object wordFalse = false;
object wordTemplateLocation = (object)templateLocation;
object wordNewDocument = (object)Word.WdNewDocumentType.wdNewBlankDocument;
wordApp = new Word.ApplicationClass();
doc = wordApp.Documents.Add(ref wordTemplateLocation, ref wordFalse, ref wordNewDocument, ref wordFalse);
doc.ActiveWindow.Activate(); // Otherwise Word complains a document is not active
object oWordBasic = wordApp.WordBasic;
oWordBasic.GetType().InvokeMember("RemoveWatermark", BindingFlags.InvokeMethod,
Type.DefaultBinder, oWordBasic, new object[]{});