Jess could have easily broken the time loop by simply not killing her earlier self and letting that version of her get onto the ship. Jess, a few extra hours older, would then pick up where she left off with lessons learned about motherhood.
I'm sure that's what she would have done if she had had a chance to remember the previous loops...
OK, so the car's trunk could easily have wound up opening during a multi-roll crash with air time. But how did the corpse in said trunk get out of the zipper bag it was stashed in?
Unless that's not the body from the boot, but the Jess who was driving the car and Jess is actually looking at her own body, before leaving with Charon the cab driver.
The Jess driving was in the shorts outfit, the body from the trunk (shown on the pavement at the crash scene) was wearing the dress with paint on it (just rewatched the movie).
Why does Jess believe that killing her friends will save everyone? There were other ways she could have saved them without shedding any blood. (Word of God says this ambiguity is deliberate.)
It was explained near the end of the movie. The Jess who got knocked overboard during the first loop reasoned that since killing them all causes the loop to reset, once she killed them all, she could meet the new versions at the boarding bay and convince them against boarding the ship to begin with, which would spare their lives. So it wasn’t killing them that she thought would save them, just that it would allow for the opportunity for them to be saved at the start of the next loop.
Jess seems constantly confused and makes many illogical and rash decisions. This all becomes perfectly justifiable, when you remember that she is in a time loop, has suffered several injuries, including a car crash, hits to the head, falling off the ship's side into the ocean and nearly drowning to death. Most likely she was in a state on concussion for her entire screen time. She is also severely sleep deprived. All the rest she gets is a few hours of sleep in the yacht and she doesn't get much to eat on screen except a drink on the yacht.
During the time loop several things keep on piling up in various locations without resetting at the end of each cycle. Eventually the massive piles are bound to change the outcome. The mountain of corpses on the upper deck will eventually spill over the railing and cover the whole front deck, and their combined weight might even sink the ship further away. The broken necklaces will eventually fill up the space under the floor preventing further necklaces from breaking and falling in. Finally the pile of dead seagulls at the beach will eventually grow into a mountain, which will disrupt traffic on the road Jess is supposed to drive and therefore she will not be able to crash with the truck. So eventually Jess will be able to save her son from the deadly car crash, although it will take awfully many cycles.
All the suspense and plot twists might eventually obscure a plain fact from the viewer, which is that the only villain or antagonist in this film is Jess herself, and everything we see onscreen is her own doing. Not a great revelation per se, of course, but the film might actually yield itself to a wholly new reading once you try to watch it with this always in mind.
