he following contains spoilers for One Piece Chapter #1015 by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English from Viz Media.
One Piece Chapter #1015 finds Luffy in a dire situation following his battle with Kaido. In the previous chapter, Luffy was tossed from the top of Onigashima, and into the sea below, after a severe beating from Kaido, who declared his use of Conqueror’s Haki clumsy at best.
That left Luffy in a precarious situation due to the weakness of all Devil Fruit users -- an inability to swim -- and no one is nearby to rescue him. Just as the alliance begins to succumb to despair, Momonosuke announces that Luffy is alive and determined as ever to defeat the Emperor of the Sea. The rubber pirate is still sinking to the bottom of the ocean but, thanks to an initially goofy set of circumstances, Luffy could yet return to battle.
How Luffy Survived the Battle Against Kaido
Previously, Kaido took this opportunity to crush the spirits of the alliance. He announced to all of Onigashima that he had defeated Straw Hat Luffy, and he almost succeeded in destroying everyone's resolve. However, in Chapter #1015, Momonosuke inspires his allies, announcing that Luffy still lived.
Throughout the battle, Momonosuke had fed information to the raid alliance as he received it from Luffy’s emotions, which he was able to feel through the Voice of All Things. However, it appears that during Chapter #1015, Luffy was able to communicate more than his emotions to Momonosuke: As he sinks to the bottom of the ocean, Luffy reassures Momo that he will beat Kaido.
How the Heart Pirates Save Luffy in Wano
Not only was Momonosuke able to hear Luffy, but, in the final pages of the chapter, the Heart Pirates, in their submarine, come across a sinking Luffy. One asks how he could talk underwater, implying they can also hear him through the Voice of All Things. That may seem like an absurdly convenient way for Luffy to be saved, but the Heart Pirates' big rescue was set up at the very beginning of the raid.
Following the initial skirmish between the Beast Pirates and the alliance, Law sent the Heart Pirates under Onigashima to take seven of the nine Akayaza to the rear of the island. That was due to the tides making the back of the island unapproachable by conventional ships.
After dropping off the samurai and their captain to fight Kaido, the submarine descended once more. The last readers had seen of the Heart Pirates was in Chapter #997 after Kaido, using his Devil Fruit abilities, picked up Onigashima. At that point, the Heart Pirates could no longer surface because of the debris raining down from the island, a handy explanation for why they were still beneath the waves, and not in the fight.