Hiccup is saving Jack.
Hiccup didn’t see Jack fall, but he heard Toothless crying out when Jack vanished beneath the waves. Cursing, Hiccup ducked under a ball of fire, pressing his heel back at the same time Toothless began to turn so they could duck under a Nightmare streaking at them. As they turned, Hiccup looked down at the churning ocean, and just barely made out the ripples where Jack must have hit.
“Come on buddy!” he shouted over the sounds of battle, Dragons clashing with Nightmares above the shores of Berk, the Guardians leading the charge. He leaned forward, and Toothless matched him perfectly, diving down to the shoreline, landing with a heavy thump on the sand. A few feet away a group of Vikings and the giant rabbit (he’d introduced himself as the Easter Bunny, but Hiccup was still having a hard time wrapping his head around it) looked up from their work loading a catapult with exploding eggs, then turned back to their work. Hiccup ignored them as well, unclipping himself from Toothless and hopping down.
“Wait for me here, Toothless,” Hiccup ordered, and then without waiting to see if the Night Fury would obey, he charged into the ocean.
“Oh man, that’s cold!” he yelped as he got waist deep into the water, but he couldn’t allow himself to falter for long. Taking a deep breath, he dove under the surface and started swimming as hard as he could in the direction he’d last seen Jack.
The water was dark and cold. No matter how hard he strained his eyes, Hiccup couldn’t make out anything in the murky waters. He came up for breath several times, always diving back down despite the panic that was building up at the back of his mind. How long could Jack go without air? He was an immortal, did he even need to breathe? And if he didn’t, why hadn’t he come back up yet?
Then, just as Hiccup was about to give up in despair, a pale beam of light pierced the water, as if the moonlight itself was marking the way. Hiccup could just make out a pale face and hands, but it was enough. He swam.
When he reached Jack, he wrapped an arm around his chest, and started kicking his way to the surface. Thankfully the moonlight was still there to guide him or he might have gotten lost and disoriented in the dark. He was hindered by his artificial leg and Jack’s dead, but he didn’t let that stop him.
When he broke the surface, coughing, Toothless was already paddling out to meet him. The Dragon purred softly as Hiccup grabbed his side, and started towing them back to shore.
Hiccup looked down at Jack’s pale face, the Guardian tucked tightly to his side, and he thought to himself, ‘Don’t worry Jack. This time, I’ve got you.’