There’s been a slight mixup with the DLC, which has now been ironed out. Those of you who have already installed Zubmariner through GOG Galaxy will now notice a message telling you that the DLC is not compatible; do NOT be alarmed! Regardless of the message, you can normally launch Zubmariner from the DLC’s card and now from your Sunless Sea card as well.
Everyone who’s using the offline installers or hasn’t yet installed the DLC through their GOG Galaxy client, feel free to disregard this notification. Now, what are you all still doing here, zailors? The Unterzee awaits!
Source:: GOG – Good Old Games