Tomás’s answer to “I believe the ending can be interpreted different ways. What did it mean to you?” > Likes and Comments

Yep, absolutely. The boatman's function is to separate troubled relationships.

I am pretty sure that it is Axl that can't forgive Beatrice for he infidelity.

So maybe the boatman is Beatrice's long-ago lover and he will spend her last days with her on the island.