I think that the Sea Kings still have to come out, and I think its more plausible to have them stop Noah then to have Luffy do it. I could be wrong. I usually am.
Also, there are the huge chains on Noah. It would be awesome to have the them pulling the ship, and the case could be made that Oda is foreshadowing with Momo(did I spell that right?). Noah would definitely be able to take everyone up to the surface, and with all the emphasis put on Queen Otohime, I think their reaching the surface is inevitable. Plus the King has made it clear that the ship is important in fishman prophecies.
If this much is true… where would the fishmen go? I think its a stretch to think that the human world would be a welcoming place for them. I don't think SA is safe, neither is the New World. Personally, I would love to see them go to the island where Luffy trained. It's away from humans, but on the surface, and the only way to get there is through the Calm Belt. Once Shirahoshi can use her haki to control Sea Kings it will be a cinch to get there.
Also, Joy Boy may be Luffy. Its a pretty accurate description of him. Plus a lot of other fishman prophecies have centered around him. So why not.