By default, your proofing comes with a standard watermark that is a copyright symbol and set to 50% opacity. If you'd like to change the level of opacity, go to the Proofing icon and click on the tab labeled Settings.
From here, scroll down to the section for Watermark Visibility and there will be a drop down menu with percentages 0-100%. Open the drop down menu and select the percentage you'd prefer.
0% will turn the watermark off, and 100% will make the watermark completely opaque.
You can also choose to have your watermark show only in the large image view, or in both the large view and grid view. This is also the area where you can upload your custom watermark if you wish.
When your watermark preferences have been selected, make sure to click on save changes to apply your settings to your proofing page.