The last line is a really strong line to finish the story with. The line to me it means like as he died he was going through memory lane and see everything he has ever done. So this shows that Gatsby wasnt living life day by day , this is because he wasnt really living life. Gatsby all he really was doing his whole life was chasing the girl of his dreams. Also he was living life for someone that didn't even have remorse when he got killed. This all showing that the boat he was riding was pretty much a memory lane telling him that he didn't live life to his fullest. But the boat is showing that he just moved on with his after life with all this regrets.