Luffy is unconscious, injured and drowning at this very moment, nobody even near to save him. It’s too late for anyone on Onigashima to come save him now.
I can see a few options for him getting out: either the Heart Pirates and their submarine pick him up or there’s some mythical creature in the waters around Wano that saves him.
I was reflecting on this thing. Everytime someone crashes or falls into the water around Wano (usually while trying to enter Wano) they harmlessly wash onto the shore (Whitebeard Pirates, Straw Hat Pirates, Big Mom).
I wouldn’t be surprised if the reason we have seen multiple times people falling into the water around Wano, is to set up there being something/person/animal that saves people when they fall into the water, and we have seen mythical creatures in Wano so it could be something like that, and could also be this creature is what Momonosuke is hearing with the Voice of All Things.
Another option leads to the Straw Hat Grand Fleet. Will the Grand Fleet finally join this war?
They all have Luffy’s Vivre Card and would have known that Luffy was in danger from Luffy’s first battle against Kaido. If they started sailing from then, they should be able to make it to Wano by now if they followed Luffy’s Vivre Card. I think maybe Princess Mansherry is with them and will heal Luffy a bit for his next round against Kaido.