Original Jess murdered Tommy in her home(we fill in the details of this gruesome scene... does she put HIM in the duffle bag?), then drove to the yacht, which capsized and they all died, except for Heather(who actually survived and that's why she disappears after that point, I thought this was an interesting twist). Here, they enter Jess' Tartarus, or personalized hell. They get on the ship, friends die, and the next group boards, yada, yada, until 3 "Tank Top Jesses" exist, and that's when the alternating pattern sets in.
They get into a pattern of 1 "masked Jess" dying on the ship and 2 making it back for a chance to save her son(including the Jess we are watching).
So the real looping pattern in her Tartarus is(depending on where you start from): 1) "Tank Top Jess" from ship intervenes and stops "sun dress Jess" from killing Tommy, Drives toward yacht, crashes in car and he dies again anyway. Next "masked Jess" falls off ship and returns home. 2) returning "Tank Top Jess" from ship intervenes and stops the next "sun dress Jess" from killing Tommy, Drives toward yacht, crashes in car and he dies again anyway. Next "masked Jess" gets killed by axe, and doesn't make it back to intervene.3) Without intervention by "Tank Top Jess", "sun dress Jess" murders Tommy, changes into tank top and jean shorts, because she gets blood on the dress, drives to harbor and gets on yacht. This is how the keys get on the ship. Next "masked Jess" falls off ship and returns. She can never fully get rid of her mean self.
An interesting note regarding Mean Jess: If the loops all begin at home(This is where all the Jesses spawn from) and continue to eventually form the looping pattern we see, The Jess we are watching is really one of "sun dress jesses" from the end of the movie. She truly is killing herself, and not just a version of herself. She is guilty of the same murder, because that would have been how it all started for her too, before the loops began.
Support for this explanation:
Tartarus fits: Her particular Tartarus, or personalized hell, would be fitting to the crime. Jess killed her son for a second chance, at freedom(the birds that follow her), a better life, etc. originally. So, kill Tommy to escape the burden of him, kill everyone to get back to Tommy, endlessly. The violent nature of the Tartarus suggests the violent nature of how she should have earned this Tartarus in the first place. The different sides of her personality continually battle over these two desires, and oddly, both conclusions take her to the harbor over and over again. Victor says about Sisyphus, "That's a pretty sh*@! punishment, what did he do?" We're supposed to ask, this is a pretty sh*@! punishment, What did Jess Do?
Referencing 237: 237 is her house number. This is a construct from the real world that is projected into her Tartarus. It's a tribute to room 237 in the movie "The Shining", about a father going crazy, and attempting to murder his wife and son(also a repeating story, where the father kills/attempts to kill his family). 237 was a room they were never supposed to enter and the heart of the evil in the hotel. So, coincidentally, she not only enters room 237 several times, but lives there, too.
Losing it: When "Tank Top Jess" goes back and sees "sun dress Jess" at her home(at the end of the movie), "Tank Top Jess" appears unnerved and ashamed, deep foreboding music starts, she knows something horrible is about to happen(murder). The paint spills because the boy sees "Tank Top Jess", then ringing the doorbell buys her time when "sun dress Jess" would have lost it and killed her son. You can see "sun dress Jess" really losing it at this point. With this extra time, "Tank Top Jess" runs to the shed, grabs a hammer and bashes in "sun dress Jess" head, without hesitation.
The Sun Dress: At the beginning of the movie, Jess is in the sun dress and ready to go. "Tank Top Jess" startles the boy and he spills the paint. "sun dress Jess" cleans it up, and gets "blue paint" on the dress, so, we have to assume she would have changed into the jean shorts and white tank top that we see her wearing in all the loops. Since only 2 of the 3 "Tank Top Jesses" make it back to the beginning, Tommy wouldn't have spilled Paint in one of the loops, and wouldn't have intervention from "Tank Top Jess". So, 1 of the 3 instances would be "Sun dress Jess" boarding the ship. What would have caused her to change the sun dress in absence of the spilled blue paint? She murders Tommy(likely in a messy way that required her to change her clothes, either through the murder itself or the cleanup). We also see Tommy pointing at the messy blood on the window, and at the scene at home, and says, "Blood!" at the end as another clue for us.
The car keys: If she always got in the car crash, from the first cycle, where do the car keys come from on the ship? They would assuredly be left in the ignition of the crashed upside down car by the 'spirit Jess'. 1 of 3 Jesses murder Tommy and drive to the harbor. This is how the keys are re-introduced on the ship. By watching the pattern, they are dropped by "Tank Top Jess" ahead of our Jess(coincidentally "mean jess"), Picked up then dropped by our Jess, Picked up by next "Tank top Jess", then the pattern changes slightly, so she doesn't drop them. It's not revealed where the keys go after this, but it's not important after that. "spirit Jesses" arrive by taxi, after the car accident, so they don't have keys.
The Seagull: The seagull is interesting bird as a symbol in "Triangle". It's a bird, so it's a symbol of freedom, yet at it's core, it's a scavenger. The seagull "follows" her throughout this journey. Pay attention to how she "interacts" with the seagull as "Triangle" progresses. She starts with a pleasant smile as she looks up at the first one while in the yard. Then, neutral on the yacht, on the ship she's looks a little annoyed by the squawking of the next one, Then, the seagulls start to "feed on the remains" of her friends, finally She kills the seagull with her car. So, the clue: Her desire for freedom was ultimately intertwined with death.
Rotting food: The food rots to remind us that even if there was no violence(perhaps the original cycle), or she tried to somehow stop the violence, the friends would all starve, and die anyway. It's a fail safe to assure the loop continues, but I believe the chosen path of violence is a reveal about what she did to get into the Tartarus, something bloody and violent. Same thing that got her in is the same thing that gets her out, rinse, repeat cycle.
Into the... Grayness?: Two points in the movie go to a sort of gray hued effect. The car crash and the overturned yacht. Depending on the version of Jess you're following, one of these two points are crucial in terms of where they "die". Since original Jess was clearly not influenced by the copy jesses, she would have originally died in the overturned yacht.
Toy boat in the pool: A red herring to question whether the overturned yacht is a manifestation from reality.
The Clock/Watch: 8:17 points to a realistic time that they would have reached the point of the car crash originally, and died. The second hand is not moving on the watch when she compares it to the clock, the clocks on the ship never change. It DOES NOT make sense as the time of his death at home, in fact, there is a clock shown over the head of "freaking out sun dress Jess" at her home that reads 7:15(this tells us time itself isn't stopped in her Tartarus). When Jess is packing the bag at home, granted this is "Tank top Jess", the watch still reads 8:17. With the connections to "The Shining", and a movie THIS clever with twists within twists, I believe that the clock/watch connection is a construct from the real world... like her house number being the room number, the song playing on the phonograph and the drum coming from the marching band en route to the ship. I think her watch had broken/died, maybe days before, and it ended up manifesting itself on the ship. I think this is a HUGE red herring, a tribute to "the shining" when Danny draws Jack into the maze and backtracks out in his own footprints. The clock is Chris Smith's way of drawing us all into his maze only to get lost(and we clearly have fun in the process).
More on Time:
8:30 Jess arrives at harbor, Victor replies you're running late.
9:00 Introductions and setting sail.
9:00-11:00 "you've been sleeping for a couple hours" that's if she falls asleep almost immediately.
11-11:30 wind died down, storm hits...
11:30-1:30 going from soaked and overboard to bone dry and waiting for a ship.
1:30-2:00 Ship shows up, they walk around and end up in the Ballroom...
Yet on the ship, "what time you got"... "11:30" WTF???
If time is telling us anything, it's the time of their original time of death 11:30, on the yacht. Since it would be assumed, the ship was the same manifestation from the first instance, her watch is broken/dead on 8:17, and being projected into the ship like the drum and her house number. 8:17 is a huge red herring.