I think he made it and here is why.
After watching it for the first time I had questions about the ending. For me the whole movie was a bit frustrating. The viewer is not given much back story but there are plenty of subtle clues on the second view. More on that at a later post.
Here is my take on the ending. There is a boat in the end with a light which has a white hull visible with moonlight. The fact the light is not that bright makes many viewers think the vessel is far away. It also clearly is not a huge vessel like a container ship more of a large fishing boat. After setting the raft on fire just about anybody would admit that is a scene you never encounter that far out and would be compelled to investigate understanding time is of the essence. As he sinks underwater you can see the size of his burning raft against the size of the rescue boat. It as a small fast tender for the larger fishing vessel which is not that far away. The lights shining down in the water show they knew to be looking for a man in or under the water. Anyone would understand that this just happened a few minutes ago and any survivors would have to be in the water. The film makers and Redford were delighted to find the ending had become a bit ambiguous to some viewers as it did for me on the first view. I think they were just looking for a clean ending.
bob