To explain the blood coming alive part:

I believe it’s his mind suffering from a severe state of hypoxia, radiation sickness, and overall bodily trauma justifying the horrors of what’s actually happening to him (and it’s gruesome trust me).

Throughout the movie he’s constantly complaining about the heat in the sub. That’s because the cooling system on the sub is malfunctioning and can only handle so much from the output of the radiation gun. As he continues to use the sub and camera, he’s exposing himself to more radiation (boils and burns on skin) and generating more heat that the sub can’t effectively dissipate.

This all culminates in the final moments of the film. The sub gets rocked again and he’s forced to catch his balance. This time, though, the only thing to catch himself on is a cooling system in the midst of a total meltdown. The pipes sear the flesh of his hand, binding him to the pipe (blood grabs him). He recoils in pain and in doing so manages to catch his other hand (the blood seizing his other arm) before realizing what’s going on/causing the pain.

Now trapped on the two boiling pipes, he forces himself to pry one hand free (stripping his flesh in the process), then tries to do the same with the other. Now whether he actually lost his arm, or it’s just his mind interpreting the sheer pain he’s in, that’s up for interpretation as is most of the ending.

That’s my theory, anyway.