The movie is running in Loop, on the life of main character - Jess.
If I am correct, she can only see 1 day, the day she boarded the ship since next day it would be the same too.
no, she can see 'all' the days, hence she knows what's happening.
But it never mentioned what happened to other Jess(es), who have same caliber to be saved.
all the Jesses want to live... so, they have to kill the redundant to continue the loop. and the movie clearly shows what happens to other Jesses.
Triangle is a refernce to Bermuda Triangle... hence the 'ship'. a plane/helicopter would be too ungainly.