The Last Voyage of the Demeter has a huge amount of wasted potential. The setting is atmospheric and the cast is decent. The real downside to it is that it shows way too much way too early. The suspense and mystery is killed by completely unveiling the "bad guy" in the first third of the movie and it unfortunately devolves into a B-movie adjacent creature flick from there on.
I'm not one to complain about characters making dumb choices for the sake of the story in a horror movie, but I think this one really turns it up to eleven. I don't recall a single film in recent memory that had the main characters make such consistently wrong decisions throughout and it kinda turned into eye rolling in the last third.
Overall I really wanted to like it, because the setting and theme was intriguing. It's not a horrible movie by any means, but it's a shame to see how close it came to being good, just to end up being pulled back into mediocrity.