The role of the boatman is the most easy part. He basically wanted to help them because he was kind by nature. It is also subtly hinted that, he felt that Lord Ullin's daughter was beautiful and wanted to help her. He did not accept any money and helped them as a good deed. He was the person who helped them cross the stormy sea (unsuccessfully). In the end, it is not mentioned whether he died or not. But, as the 2 main characters die, it is assumed that he died.