User blog comment:Monkey.D.Me/WHERE ARE THEY NOW ???/@comment-3075024-20111221185335

Thou this is not a theory, I just wast to throw this in. I think I have found a look-alike mermaid of Nami 

http://i1213.photobucket.com/albums/cc470/monkeydme/Capture.jpg

have a look and you be the judge. Orange hair, Green-White strap bikini and blue fin .. .. .. i think this is the gag that we saw in the beginning of OP when Luffy thought that Nami is mermaid and thats why she knows Arlong.