You'll be happy to know that the watermark is a repeater that can easily be removed from the template editor the moment it's imported to your content library!
To delete a repeater, enter the editor page for any proposal or template you see it. Once in the editor, click the “repeaters” button found on the right-hand contextual menu:
This will set your editor to “repeater mode”:
While in repeater mode, locate the "sample" image and click on it to select it, then hit backspace/delete on your keyboard. That will delete the watermark from every page in your template. Here's a quick GIF to illustrate: