So I agree with JR. Use the free actions that already exist. You can modify the scripts to make them work the way you want.

But if you want to do this yourself, why are use a watermark?  The watermark is a rather top heavy way to do this. It has an issue your code doesn't deal with. A watermark creates an OCG layer. When you create the watermark on the next page, you replace/overwrite the last watermark OCG. A PDF file can have only one official watermark. You can read about it here. 

https://acrobatusers.com/tutorials/create_use_layers/

A much easier way to accomplish the same task is to use a form field. At the end the script can flatten the fields if you want them pushed into the page content, or the field can be set to readonly.