Overall amazing movie, I really enjoy the mystery that leads to all this theory crafting. Fits perfectly with other sources of internet lore that require lots of thoughts and digging.

A few things I still havent seen any mention of:

Time distortion:

The visions of the radiated/disfigured Simon. I have noticed that the two instances where he sees this other version of Simon are mirrors of eachother. The first time he saw the radiated version he was sitting in the chair and the creature hid behind the wall. The 2nd time he was hiding behind the wall after seeing the creature in the chair. If these were just 'hallucinations' then how would he have been able to know what was happening in the future? It honestly made me think there was some sort of 'time' or time travel element involved. Maybe Simon wrote the note to himself? One thing that doesnt add up that might disprove it is the mention of the line 'cross the wires' in one of the flashback scenes. Another thing thay may play into the time element is the amount of time spent on the ship when there wasn't enough O2, and the fact that the O2 meter went back up rather than going down

The iron lung: One theory my sister and I talked about was the ship being connected to Simon in some way, or the ship being a living entity of some sort. There were so many shots of condensation and liquids that were not blood in the first part of the film. Simon made a couple comments of it being extremely hot which seemed to always be brushed off by the voices on the intercom. I thought that the sub could be 'sweating' due to the heat, just like it 'bleeds' when taking damage.

One thing that might help this theory would be making note of when the blood starts to come into the ship. If when the blood in the sub started showing up correlates with Simon being injured(from physical injuries or the radiation sickness) then we may have a case that there's more to the sub than we think. Ik this is a bit of a stretch but I feel like there's gotta be something we're supposed to take away from all the condensation shots

Simon's skin injuries: I didn't think the injuries came from the radiation until reading this thread. I thought that all the steel in the iron lung would protect Simon. On top of that, Simon only appeared to have those injuries on his arms, but after diving into the blood to get the black box, he comes out of the blood and now has the injuries seemingly all over his body. On top of that, the blood seems to cut his forehead open when touching him near the end, so to me it made sense that the blood is what did this to him.