There's one problem if the fishman are going to the surface: how can pirates get in the New World then? I mean the only way they have is passing through FI, though if the fishman leave it's going to be quite a problem.
They don't really need to dock at FI, they can just continue straight through the hole underwater and up on the other side.
And a great way would be if Fishmen became integrated in the community, there would be many living in Shaboady, and they could just escort pirates into new world through underwater, by summoning large sea animals for advanced navigation.
Right now, the fishman island is like a refugee for fishmen, it's like a barrier for the fishman. The feel safe on their island, because not many people can get to the ocean floor, and the fishman can easily defend themselves while their on the island.
So they stay on the island to feel safe, but the island is also the thing that is keeping them from integrating with the humans. It's rare for humans to see fishman, thats why people capture them as slaves and such, and they also discriminate the fishman because they are afraid of them, afraid of the unknown.
But if they were forced to live with the humans, the discrimination wouldn't stop right away, but it would disappear gradually, because fishman would become a part of the human community, and it wouldn't so unusual for human so see them.
I think that fishman will live above ground, with Shirahoshi protecting them with the ability to summon sea kings xd (I'd like her to join straw hats cause thats an awesome ability and she would be a great match against Sanjuan Wolf with that ability, but can't see how she would fit on the ship, and can't imagine her just swimming besides the ship
) She'd protect the fishman island population till humans got used to living with fishmen.
