Message in a bottle. My favorite moment.
Numerous clues hint that he's not ready to face something--that he ran when he should have stayed. My best guess is that he's driven by guilt or shame.
My favorite moment in the movie was when he grapples with throwing the bottle in the ocean. He hesitates as if realizing that throwing the bottle gives truth to the words he wrote. His hesitance implies that maybe he's not really sorry or that his words aren't truly genuine. The irony is that no one, especially his wife (or whomever he wrote the note to), would ever find it. There's a subtle humor in this--that lost at sea, his greatest struggle is not to survive, but to admit his faults.