Thank you for the savegame, FetterBruder, we have forwarded this issue to the dev team.
GU 5.3 was the last update we released (17 October) and no further patches were/are planned till the release of GU 6.0 in early December in order to fully focus on the development of the new content and fixing of known issues (with a playtest for it going on right now).
As soon as we got more details about GU6.0 and/or this specific issue with the Sunken Treasure questline, we will let you know.