Just watched Pirates of the Caribbean: On Stranger Tides, and there's a part that wasn't explained. I'm going to give my theory on what happened, which means, spoilers alert.
So, there's the part where that guy asks the mermaid for forgiveness, and she then kisses him and brings him under the water. What then happened? Here's a nice little theory: She brought him to some magic-thingy that turned him into a merman. Since he was once human, and doesn't want to kill people, he teaches the other mermaids not to kill people. They later make him their king. He changes his name to Triton, and has a daughter named Ariel.