docbream-2017
Hi
Personally i have not used any watermark plugin for sites because it was not in my requirements, You can find the watermark extension on JED here and choose the best one as per features.
If the extension work with Joomla articles, module it will also work for the JA Focus.
If you are going to buy any paid extension i suggest you to confirm your doubts with extension provider about its working, as some time extension only work with core Joomla parts not with custom modules and custom fields.
Regards