My interpretation

First, the movie is not "Robert Redford against sharks and storms". It is about the iron will to survive. Many a time, while watching it, I asked myself "And now? What he will do after this dire thing?" Only to see Redford come out with another out of the loop idea (cue: how he manages to get potable water after the main reserve is contaminated).

If anything, I saw it as religious metaphor. Redford battles the elements until the very end. Then he surrenders. It is like if God battered him until he admitted his shortcomings in the letter.

IMHO Redford dies when he let's go and sinks in the dark ocean (a metaphor for death in many cultures, as people like C.G. Jung noted). Then he sees a blinding light, swims towards it, and an hand appears to save him. The symbolism (still IMHO) is almost too obvious.

Just to make things clear, I'm a spiritual person (I believe in the existence of the Soul) but not a religious one. Yet I can accept a different point of view.