The movie just shows us the never ending loop in which Jess is stuck seemingly forever. What actually happened that left her in this loop is never shown. Only subtle hints are given throughout the movie.
There is no starting point to the loop. That just seems so to the viewer. I would say that the instant she has the car crash with her son was the starting point. In fact, it's possible that she was an abusive mother who actually killed her son in a mad fit of rage. When she arrives to the dock for the first time, she says that her son is at 'School' as she is unable to come to terms with what she has done. The whole movie revolves around the 'never ending hell' she goes through because of her guilt. She keeps thinking that she can do something to 'save' her son. I believe that the cab driver also has a role to play. There was something suggestive in the way he says stuff like
I'm just a taxi driver
So, I will keep the meter running? You promise you will come back,
Maybe he 'represents' the devil, waiting for her to accept the truth and be free from the loop of constant suffering?
She kills the 'original evil mom Jess'. This also represents her inability to accept the truth. She later tells her son how things will be better from now on.
Another point to take into consideration is the fact that initially it seems as if the time loop is restricted to the Ship. But the scene where she discovers the dead birds in the pit, shows that she realizes that it's her whole life, for that particular day,and not just the Ship. She can't just run away from it.
Time on Jess's watch exactly matches the time on the Ship. This is not the case for anyone else. Another indication that the whole time loop is just about her.
The Jess which stabs Sally and Downey can definitely exist. The events will happen as they 'should' unless Jess does something different. Everyone else will act the same way. The scene where she is confronts 'the new Jess' with the gun but is unable to shoot, this new Jess runs away and is shown killing the 'Masked Jess'. The movie never shows what happens to her. Maybe she is the one who ends up stabbing Sally and Downey.
So, to summarize my answer, in my opinion,the movie starts out with an impression of being a strange sci-fi/time-loop movies (which it kind of is), but the part where she return to shore and thereafter indicates something more which is going on, apart from the time loop on the Ship. That's my view.
PS: Check out the story of Sisyphus.