WARNING: The following contains spoilers for One Piece Chapter #1015 by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English now from Viz Media.
Chapter #1015 of One Piece finds Luffy in a dire situation following his battle with Kaido. In the previous chapter, Luffy was tossed from the top of Onigashima into the sea below after a severe beating from Kaido, who declared his use of Conqueror’s Haki clumsy at best. This left Luffy in a precarious situation due to the weakness of all Devil Fruit users, an inability to swim, and no one in the immediate vicinity available 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 Emporer 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.
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 helps inspire his allies, announcing that Luffy still lived.
Throughout the battle, Momonosuke had been feeding information to the raid alliance as he received it from Luffy’s emotions, which he was able to feel via the Voice of All Things. However, it appears that during Chapter #1015, Luffy was able to communicate more than just his emotions to Momonosuke as, while he sinks to the bottom of the ocean, Luffy reassures Momo that he will beat Kaido.
Not only was Momonosuke able to hear Luffy, but, in the final pages of the chapter, the Heart Pirates, in their submarine, come across Luffy sinking. One asks how he could be talking underwater, implying that they can hear him as well through the Voice of All Things.
This 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. After 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. This was due to the tides making the back of the island unapproachable via conventional ships due.
After dropping off the samurai, in addition to their captain, to fight Kaido, the submarine descended once again. The last readers saw 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 due to the debris raining down from the island, a handy explanation for why they were still under the ocean and not in the fight.
Now that the Heart Pirates have Luffy in their sights, it looks like they will be able to save the Straw Hat, but whether he will have the strength to defeat Kaido and liberate Wano is now the most pressing question.