Robert Redford's sailor doesn't talk to anyone because there is no one there to talk to and there is no contrivance to make him speak. He attends to the task at hand, which is what a person in this situation would do.
If you've seen the preview you know Redford's (and his character is not named) character's boat strikes an errant shipping container. From there he works on fixes and plots courses while battling the elements and plain old bad luck.
One amazing thing about the movie is that it never lags and it manages a sort of low-key tension throughout. Redford never has a real moment of rest, of comfort, as he tries to save himself. Director, J.C. Chandor, foreshadows some of what is to come in the film at the outset. The audience knows what the man on screen does not. Truthfully you know if you saw the previews but it is artfully done in the movie.
One other thing about Redford--he looks good for a man his age but he looks at least CLOSE to his age. This is something aging leading men (or former leading men) should take a cue from. When you get older and you get Jean-Claude Van Damme-style plastic surgery it really doesn't prolong your career. It just makes you look like an alien.
This film curiously mirrors the recent film, Gravity, also about someone marooned and trying to survive but in a slightly different environment. Gravity is spectacular looking and keeps the back story spare. But it is incredibly detailed compared to the story of All Is Lost. We can glean some details from little clues throughout but we only have what happens on screen to go on--there is no talking, no photos of loved ones and no doomed side-kick. It is all Redford and the sea and curiously that is more than enough.