Folklore about mermaids?

I just went and saw the new "Pirates Of The Caribbean" movie and was confused on one small bit. The religious sailor who was eventually saved by the Mermaid, said something about the mermaids being related to the condemned souls of those taken out by the flood from Moses's time (Or thats what I gathered, he mentioned a flood I know that for sure). What is the folklore concerning that? I've looked and looked for an answer but found nothing close to what the movie hinted at. I really want to know because that was an interesting way to think about the people that were drowned in the flood. Anything is appreciated!