Triangle (2009) -- Jess goes boating with a group of friends. The boat capsizes and they are picked up by an ocean liner. The film becomes very weird & circular from there. The group is picked off one by one but this isn't your typical slasher.

It's like watching a film made up of layers of reflections in a mirror.

Interesting and unusual concept.https://www.youtube.com/watch?v=17XqBdCiHOI