Since I've been lurking on the forums a lot and never seen a thread mentionning that (and sorry if there is one, I actually searched for it and couldn't find anything.), I thought I might suggest it. Don't flame me, please, it's my first thread ever.
I thought that Franky might be part merman or fishman. He seemed to have great… "swimming skills" even when he arrived to Water7.
The day he arrived, he was already wearing his Hawaiian shirt. I know this might be pointless, but every fishman we've seen so far that I can remember (Exept Hachi, Chu and Kuroobi) were wearing hawaiian shirts. Maybe that's just a trend in Arlong's, but Tom was also wearing one. Getting kind of used with how Oda acts, the fact that Franky has always been wearing that might be more significant that you'd think. Or I'm just imagining things.
Also... he didn't seem too suprised with Tom's...appearance. I know this is One Piece, and that we're in the middle of the Grand Line, but one would think that a huge santa-looking fishman with big pointy teeth would scare little kids. Franky just called him as "ossan", which is a rather casual way to call a more or less aged man, if my memory serves me right ._.;
That's all I can think of for now... so tell me if you think this could make sense, or if I'm just crazy. :/