From the Letterboxd Twitter profile: "All aboard for Showdown № 5: Boat Life. Narrative or documentary films set all — or mostly — aboard or around boats. No subs or spaceships!"
I'm not sure that I have seen ten films that are primarily set on a ship that I would consider good. At least I can't think of ten right now. So to make a ranked top 10, I have added a few okay ones, and some that just have a lot of ship in them but also a lot of land!