My take:
The entire movie is a metaphore about living with addiction, specifically alcoholism. I don't think any of the film's events are literal.
These thoughts are a bit scatter-shot since i just saw the movie a couple hours ago:
Pattinson got sick of trees. Forests of brown bottles. The man who's identity he is carrying? That's the man he was before he was washed away by the logs. He stood there and did nothing while he watched himself drown.
I think Pattinson is trying to dry out. 2 weeks to detox. He's isolated himself with his addiction, represented by Dafoe's old sea dog who pines for the days he sailed free on the sea and harrasses and berates him all day long, barking at him and demanding respect. At night, his addiction cozies up and tries to be friendly, tries to tempt him. To act as a balm for the "long night ahead."
Only two weeks...
The one-eyed gull represents the spectre of Tom's former addiction, or maybe his fear of addiction. They host the souls of "dead sailors" drowned at sea. Sometimes sailors call the ocean "the drink." After he chooses to eliminate their warning he gives into the literal drink. The weather changes.
He missed the boat right at the end. Downward spiral. Storms blow in. Ocean gets aggressive.
"Recommends severence without pay" despite working so hard? = all the pain and suffering of detox just to slip up at the end
The Lighthouse's light is the ultimate high the addict is forever in chase of. Knowledge of the Gods.
The final scene of Pattinson getting eaten alive by gulls is a specific reference to Prometheus, who's liver was eaten by eagles, only to be regrown and consumed again each day. In this case, the booze is eating Tom's liver.
The reason it turns out Dafoe and Pattinson are both named Tom is because they are two sides of the same person. It's living with your addiction. Pattinson wants to be straight and sober and Dafoe wants to get drunk and worship the god (the high) in the lighthouse. At the end, he has his addiction on a leash and he thinks he has control of it, thinks he can bury it. But he can't and it attacks him and when it does he is basically on a damned path.
The duality of the mermaid, beautiful and monstrous siren of the treacherous ocean, plays into this too. I'm still not clear on the symbolism of jerking off to the mermaid totem but Tom think's he's free of Dafoe's control when the human half that lives on land splits from the fish half that lives in the ocean.
My thoughts are still evolving but this is where I'm at right now.
"Hypnos is the
God of Sleep, which he induces with purest opium smoked through a horn. Potentially also the
God of Hyppies. He could also be the
god of comas and dangerous
addictions, for he works with his deadly brother Thanatos, the
God of Death, for Hades Underworld Operations Inc."
Lemme know what you guys think of what I wrote in this post...
Cigarettes, i believe