What do you mean? Does it show the watermark when you open the file, even though you added the script? Where did you place it, exactly?
The same is happening to me... Maybe using this watermark is not the best option. It seems to be a bit problematic. You can remove it and use a button field, instead. Make the fill color transparent and set its label to "COPY". Then set its appearance to hidden and adjust the print code to the following (let's say the button is called "COPY"):
this.print();
this.getField("COPY").display = display.visible;
this.print();
this.getField("COPY").display = display.hidden;
That way you won't need to add any additional code to hide it when the file is opened.
Oh, and you can duplicate the button to all pages by right-clicking it (in Prepare Form mode) and selecting Duplicate Across Pages.