Why does everyone assume he'll be there just because he's a fishman?
I mean, he's a freaking Shichibukai for crying out loud. A pirate. Pirates roam the seas freely, and don't confine themselves to one place… except Crocodile, but he just wanted to rule Arabasta and didn't care about anything else =P
As I pointed it out in the other thread, except for Mihawk the other Shichibukai don´t seem to bother roaming around pretty much. You already pointed out Crocodile who tried to take over Arabasta and therefore remained there. But in order to achieve his aims he remained there pretending to just run a Casino. Seeing it that Moria said Kuma is the only Shichibukai that really follows the WG it is most likely that the others also do something else to cover their true goals.
As Gorlom pointed out, Doflamingo made his comment to Sengoku that business is doing good at the island. We don´t know which island he´s talking about but clear indication that just like Crocodile he has a fix place he needs to use as a base.
And Moria also isn´t really roaming the seas. He´s been within the Florian Triangle for 10 years. So while yes, technically he is sailing not making harbor for 10 years because of your plan isn´t that much of what a pirate does. Additional to that his ship is an island itself so he practically also remains on the same spot.
So why should Jinbei´s position as a Shichibukai actually hinder him being at his home island? I think that being an underwater paradise alone makes it a very attractive place to remain there. If it is an entrance point to the New World then it also is attractive from a businessmans point of view. I mean, even non-fishmen would find that interesting, so a fishman himself who´s even a local should find that place should even moreso attractive for whatever his aims are.
Except for Mihawk none of the Shichibukai strike as people who continue roaming the seas. Which of course makes seing as they aren´t hunted by the government anymore.