Why does the ferry captain decide to help?

Huck tries to try to save the robbers, because he feels guilty leaving them for dead, he tells the ferry captain that his family ran into the wreck while traveling downriver and that they are stuck there. The man immediately gets his ferry moving to try and save them. However, before he gets very far, the wreck floats by, having come loose and sunk even further. Huck realizes that all three men aboard the wreck have surely drowned.