This contains a spoiler, so do not read past this if you do not want to know.

Leonardo has just a couple of words in the movie but his presence in the movie is vital to the ending. He appears just a few times in the movie and at the ending Steve gives his sons ring to the boy and then in a following scene he is walking with the crew to the boat. As I see it, Steve Zissou had just lost his son that gave him his success and purpose back and Steve now was passing the baton to the boy as his student, the son he never had and his successor.