A family of Microsoft word processing software products for creating web, email, and print documents.
Are you saying to copy the letter head and watermark separate and then copy the letter body afterwards ?
A document contains several independent 'layers', in effect like a pile of transparencies which when stacked produce the finished document. The header is in a different layer from the body text of the document, so you cannot copy them together.
As Doug and Suzanne have suggested you could create a copy of the document, or you could use the document as a template to create a new document. Otherwise you will have to copy the elements separately.