There are hundreRAB to choose from, but I'm going to nominate Beyond The Bermuda Triangle (1975 TV Movie) with Fred MacMurray, Sam Groom, and Donna Mills.
Back in the late 70's this was once Tyne Tees Television's prime time Saturday night movie offering - yep, a crappy 74-minute TV film. I've just looked it up on IMDB and someone has given it a decent review on there. Don't believe them - it was terrible back then, and it would play 10 times worse now !
The review says it has a low-key ending - no it doesn't, it just enRAB ! It's like they wrote every page of the script bar the last and suddenly thought, "Hey, nobody really knows what happens in the Bermuda Triangle, so let's just leave the viewers with an enigmatic shot of an deserted boat !"
Utterly dire.