Use this auto action set to apply transparent-looking effect on your watermark. This can only be apply on a rasterized layer and a watermark that consists of alpha-numeric and special characters.
Download it, and apply/start the Transparent Watermark in Auto Action Window.
If Auto Action window is missing on your workspace, just go to: Window > scroll down > Auto-Action. This will enable the window for auto action. Select the Transparent Watermark set, then click the "play" button to apply the said effect.