I saw this movie on Tv when I was a kid. I think it was made in the 1950’s and it was in color. A group of passengers are stuck on a cruise/paddle boat surrounded by fog, sea monsters are terrorizing them, and they don’t know where they are. Someone gets killed by an octopus tentacle that comes through the porthole.

Near the end of the movie they land on a swamp island and there’s this cult with a boy as their leader. The main priest is dressed in white like the Ku Klux Klan. When the passengers try to escape back to their boat the boy says he wants to go with them, but the head priest stabs him in the back and the boy dies.

Any help with this would be greatly appreciated.