@Dan:
He could gain a lot by travelling with Luffy.
Luffy's surely going to do a great thing for Fishman island by the end of the arc. Probably the thing that moves merfolk/fishmen closer to humanity than anything else. That's something Jinbe obviously supports more and more. And If he joins the straw hats, he'll have plenty of opportunity to fight the WG, the greatest enemy to Fishman Island.
Yes, Luffy will cause a great change in FI. Something to help the Fishman n Human understand each other better? Probably, and most likely so. And Jinbe will definitely support such deed.
But thats where it ends, there isnt any strong reason to go with Luffy. More opportunity to fight with the WG??? "My dream is to topple down the World Government, Fishman's greatest enemy!!", he'll be better off joining Dragon for that.
And so far each of the SH dream involves some specific, "selfish" ambition. Something they need to accomplish for themselves (yes Zoro is also doing it for Kuina's sake, but still he'll be the one to take the title right?). While Jinbe doesnt seem to have anything for himself, joining the Sun Pirates for Tiger, becoming the captain to continue Tiger's will, shichibukai for the sake of his crew, Tiger, Otohime, and his people.
If theres any sort of "selfish" dream, I think it will involve something with his first job. Neptune's army soldier, we still dont know why he chose to be a soldier in the first place right? So we'll see how that plays out