On UnVale, you can toggle watermarks on and off via the Image Details page when editing your OC.

You won't be able to see watermarks on your own images while you're logged in, but you can confirm that they're displaying properly by logging out or opening the OC page in an private browsing window.

If you're sharing a device, be sure to close all OC pages upon logging out, so that no one else has access to the unwatermarked versions of your images.