One Piece:
Even ignoring everything else, Jinbe has a mandate from Ace to watch over Luffy. He did leave him after they went back to Marineford, but that was with the knowledge that Luffy was planning on training solo for two years and that his next stop was Fishman Island. Everything so far suggests that, despite his initial resistance to Ace's request, Jinbe's respect for Luffy between Marineford and Fishman Island has grown such that he has since taken that request to heart.
Ace was watching over Luffy before he got captured by Blackbeard. Jinbei doesn't have to join Luffy's crew to fulfill that request.
Oda hasn't been at all subtle about Jinbe's joining throughout this entire arc. Jinbe has natural storylines and dynamics with several crew members, has been given a more dynamic personality and "Straw Hat-like" spit takes that had never previously existed for him, and he has been practically a full on crew member for the last few months, much the way Franky was at Enies Lobby.
Spit take? You mean when he popped his eyes out of his head like Enel and Perona (another so called Straw Hat candidate)?
It's hard to see Jinbe not joining. Unlike other past possible members, Jinbe is not royalty (Vivi, Hancock, Shirahoshi) nor is he someone like Paulie whose presence at home is extremely vital to the success of his homeland.
And what about Mr. 2, Iva, and Margaret and all the other characters that were "hard to see" not joining in their respective arcs? The only reason you're excluding characters like Boa and others now is because of hindsight.
Jinbe has already been a pirate, he's spent years away from Fishman Island, and even now he had removed himself from the people of the island even before Luffy had arrived. There's nothing tying him to Fishman Island, no reason for him to stay. It will be far more helpful to the cause of the fishmen as a whole to have Jinbe travel with a human crew as an ambassador for the species on the seas above than it would be for him to stay at home. I don't see any reason at all why Jinbe wouldn't join other than "I don't like this character enough, waaah!"
1) Unlike Chopper, Zoro, Nami, Sanji, Franky, Ussop, etc... the Straw Hats are not looking for the role everyone keeps claiming Jinbei would fill (Helmsman/Race relations expert/water rescue/old wise man/etc...). When Franky joined they needed someone to repair/make them a ship, when Chopper joined they needed a doctor, etc... The only two crew members that didn't follow this pattern were Robin (who was given a function at the time she joined) and Brooke who role Luffy had been looking for through most of the series and just finally showed up.
2) Why the hell would Jinbei continue being a pirate if he wanted to be an "ambassador" for the Fishman? I really don't understand why people keep pushing this "he can help Fishmen by being a pirate" theory around as Jinbei's motivation for joining the crew. Being a pirate is NOT POSITIVE. Being affiliated with Luffy is NOT POSITIVE. Jinbei sailing with Luffy is not going to help him be an ambassador or help make Fishmen look good. Hell at least as a Shichibukai he had a position in the military and was recognized by the World Government. The queen's actions before she died with the signed petitions were WAY more helpful than this theory. Wouldn't it be more helpful to the Fishmen if Jinbei was visiting places where discrimination against Fishmen is more abundant like say...Sabaody Archipelago and Mariejois, the main two (and probably only) locations where Fishmen slavery still happens and exists in the world of One Piece since the government officially outlaws it everywhere else?
3) How is getting One Piece/sailing all of the Grand Line helpful to Jinbei in any way? For all the rest of the Straw Hats, sailing to the end of the Grandline with Luffy helps them with some goal/dream. What does Jinbei gain for trying to reach the Grandline/get One Piece?