I got that the final cicle starts at the end of the film, not that it repeats the first one.
The jess of the ending is the jess that wears the mask and gets shot, takes it off to pretend to be the earlier version, lead the couple to the cabin an slits his throat and say's "sorry, but I need to see my son"
She is the cold
blooded killer, the one with practice. She also says, when referring to the older version Jess that she "is not who they think"
We never see what happens to that one, and she isn't explained in the loop shown in the movie. (Although the woman she stabs in the stomach is, she dies several times on the deck, this is the only part I can't figure out)
(Edit: There's an explanation for all of them. The blooded killer is the original Jess at the end of her story arc. It's her second time boarding the cruise ship and she gets killed by another Jess that has boarded twice and is watched by a Jess that has only boarded once. The Jess's that have only boarded once are the ones that push each other into the sea while wearing the mask. There are basically seven stages to her cycle.)
Stage 1: Jess goes onto the cruise ship and pushes the masked killer (Stage 3 Jess from a previous boat) into the sea.
Stage 2: Jess watches another Jess in stage 5 kill the unmasked killer that's in stage 7 while comforting the dying Sally. This Jess in stage 5 is the one she points the gun at in the banquet hall. She boarded the ship on the second boat.
Stage 3: Jess becomes the masked killer and gets pushed into the sea by a new stage 1 Jess that was on the third boat to arrive in the movie.
Stage 4: Jess goes home to save Tommy and gets in an accident and decides to go sailing again.
Stage 5: Jess boards the ship with her full memory which is the fourth boat to arrive in the movie. Stage 2 Jess from the third boat points a gun at her and she kills Stage 7 Jess from the second boat.
Stage 6. Jess hides and let's a new Stage 1 Jess from a fifth boat push Stage 3 Jess from the third boat into the sea.
Stage 7. Jess becomes the unmasked killer and is killed by Stage 5 Jess from the sixth boat to arrive.