Only Enterprise Site Admins have the ability to create and lock default watermark settings for all users on an Enterprise site. 

Step 1:  Log into your Enterprise Site Admin account and select the Site Admin Project from the Project dropdown menu.




Step 2:  Click the 'More' menu and select 'Enterprise Site Settings' 




Step 3:  Set your preferred Watermark for the site and then set the File Sharing Setting for the site